Ajin Wiki
Kei Nagai
Name Kei Nagai
Kanji 永井 圭
Rōmaji Nagai Kei
Status Alive
Age 17
Gender Male
Hair Color Black
Eye Color Red
Occupations Student (Former)
Height 168cm (current)

130cm (child)

Voice Actor Mamoru Miyano (Japanese)
Johnny Yong Bosch (English)
First Appearance Chapter 1 (Manga)
Ajin Part 1: Shōdō (Film)
Episode 1 (TV Anime)
Image Gallery
People don’t change.
— Nagai Kei

Kei Nagai (永井 圭, Nagai Kei) is an Ajin and the main protagonist of the series. 


Kei at the start of the manga.

Nagai Kei is a young man who spends most of his time by alone, studying or just keeping to himself quietly. He's voiced how much he dislikes physical activity as he "comes from a white collar background". He has short black hair as well as pale skin. He has thin black eyebrows. He's 5'5, slightly short for his age, and has a fairly normal body shape/proportions- though he puts on weight easily.

In the anime, Nagai's eyes are a dark red colour. At the start of the manga, his eyes appeared black, before later becoming a piercing red.

After falling into the care of the ajin research facility, his hair is shaved short and appears spikier. In the anime it stays the same length but in the manga it grows back to its original length
Nagai is first shown to wear a plain, white button-up shirt and black trousers- his school uniform. He has been seen in many different outfits throughout the manga/anime, usually consisting of baggy trousers/shorts and a t-shirt that sakurai (mangaka) likes to draw with an ironic design/logo depicted on them, depending on the situation Nagai is in.


Nagai was first shown with an academic, serious, almost cold personality. He would often get lost in his thoughts, leading him into getting in a car accident as well having his identity as an Ajin revealed to the entirety of Japan. Afterwards, it is revealed that Kei is a complete pragmatist and emotionally detached from others. He is very intelligent and hardworking with ambitions to be a doctor who finds treatments or cures illnesses that don't yet have one; specifically to cure his sister Eriko, who has a long- term disease that affects her mobility and normal functions, which is thought to originate from genetics. However, Eriko observes that Nagai is not thinking about her at all and is only pursuing this goal for his self-interest - but it is shown that Nagai cares for his sister.
Nagai seems to have strong personal values that he tries to live by. He demonstrates gratitude for Kaito's help and cares about his welfare. He's also able to endure torture without killing or retaliating because it wouldn't be fair if he didn't risk everything beside his life while Kaito put his life on the line. He also doesn't begrudge Satō for having killed others to free him, but does not support killing more people unnecessarily.
He is also quite sarcastic and answers things in deadpan, such as the time when he casually mentioned that Tosaki should have died too or calling Kō a blue-collar boy.
Because of Nagai's emotional detachment from other people, he'd always found it difficult to show his emotions. Even as a child, he was shown to be pretty calm and reserved to himself, but abnormally inexpressive for his age. This is the main reason why he doesn't have the best relationship with his little sister Eriko- even though he cares for her, he hasn't been able to show it very well, which has caused her to think that he is a cold-blooded jerk that is only trying to help her out of his own self-interest. This emotional detachment has not only complicated his relationship with his sister, but all of the relationships he has with his friends/family and other people.

A significant example is his relationship with his best friend Kaito. Even though Nagai is presented to be rather rational and cold towards Kaito, we later on find out that Nagai cares about Kaito more than Kaito even cared for him. With Kaito having a hardcore saviour-complex and an appetite for violence, it was his nature to go out and save Nagai again and again, risking his life and fighting for his 'friends' is a no-brainer- a need and a must- for him, even if he wasn't particularly friends with the person beforehand. So saving Nagai's skin a couple times was like a simple favour in Kaito's eyes. However, for Nagai it isn't that simple. He usually puts himself first, but has shown to care about Kaito and put Kaito before himself in many situations- not just because Kaito's a human , but because Kaito is his friend (we know this because of the way that Nagai treats most humans- he even regarded the black suits and Hirasawa as just tools). Nagai has risked not only his life, but his body for Kaito many times throughout the manga (and in the anime too). For example, when he refused to kill the researchers while being experimented on even though it would mean that he could escape the torture they were putting him through. He seldom cries throughout the animanga but he's cried for Kaito; "If I do this.. Kai might hate me" "If I don't risk everything I have besides my life, It wouldn't be fair to Kai." It's crystal clear that Nagai cares about Kaito, and leaving Kaito behind -pushing him away- was Nagai's way of keeping each other safe, and protecting him from danger. Nagai was not only pushing Kaito away to keep him out of danger, but protecting himself as well, because Kaito could easily be kidnapped and used as a tool by others to get to/manipulate Nagai. And Kaito understood this, to an extent at least, and that is why they were able to stay friends despite being separated for most of the time that they'd been acquainted.

He is ultimately calculating and utilitarian, but can be kind in important situations as well. He saves a doctor's life who helped Kei escape out of his values by shooting Sato instinctively and saying that it is wrong to kill the researchers. However, Nagai denies his empathetic side often and claimed that he only did it because the researcher sympathized with Ajin. He is shown to not be afraid to die as an ajin, as he'll just come back to life but he is cautious of the consequences of dying.
According to his sister Eriko, Kei is logical and heartless to the core and is only appearing to care because the situation demands him so. He seems to have gotten his cold demeanor from his mother. However, his mother claims that he got a bit of his father's caring side, no matter how much he tries to hide it.


Kei first died when he was a baby but was revived due to his status as an Ajin. It was never described what actually killed him, but some think that it was from perinatal hypoxia (when the baby does not get enough oxygen when being born), or a more extreme case of the disease that his sister Eriko has. The doctors who were trying to treat him believed it was a miracle, as Ajin were not very well-known (the first ajin had only just been discovered at that time).
When Nagai was younger, he saw his black ghost, later to be named yurei (or ghost). It happened when he buried his dog with his little sister. Nagai is shown to have been cold and practical even when he was younger as shown how he easily discarded Kaito when he heard that Kaito's father was a criminal. Nagai's mother, Ritsu Nagai, later explains that cutting off Kaito was necessary to become a doctor and save Eriko as she believed that Kaito was a bad infuence on Nagai.


Early manga[]

Kei is introduced walking with some of his classmates to school where he is playfully mocked when telling them that he will be studying over summer. In class, they are taught about Ajin, a subject which Kei first decides to ignore as it won't be coming on the exam, but ends up perking his interest when learning more about them. On the way home, Kei has a flashback to a time he saw his IBM during his childhood. It causes h

Kei's first revival

im to get lost in thought and consequently hit by a truck. Here, he is revealed to the public as an Ajin. He flees quickly after stunning the people around him with his scream. He hides in the woods and panics about what is to happen to him and thinks about who he should call for help. Realising he has nobody he can trust, he calls Kaito, who's number he had memorised since they were children. Kaito promptly comes to his aid and the two run away together on Kaito's motorcycle.

They run into some trouble, the first being when two men ambush Kei while he had gone to pee, but he stuns them with his scream again and he and Kaito are able to get away (his rogue IBM later ends up killing them on a railroad, unknown to even Kei). The second time was when another motorcyclist tried to capture Kei. The pursuer kicked their motorcycle while

Kei kills himself to heal his legs

driving causing Kaito to crash and injure his head, but he was only playing dead and hit the pursuer unconscious with a rock. Kei's legs had become broken from the fall, but he slices his neck to die and heals himself through being revived. The pair drive for a bit longer till they reach an abandoned cabin. They decide to take turns to sleep and keep watch, but at this point, Kei has decided to leave Kaito behind, because he fears for his safety and regrets unfairly dragging him into his own problems. He leaves the food and water with him, as well as a note, but steals his phone, in order to be able to contact other Ajin.

In chapter 69 it is revealed that Kaito was actually impressed with Kei upon finding his note saying that he had "more guts than he'd thought".

Kei in the experimentation facility.

Kei receives a call from Satō through his sister's phone (as they had kidnapped her at the time) asking to meet with them. After meeting them and showing concern for his sister, Satō decides that he isn't yet ready to join them as he still shows concern for humans. They then knock him out, leaving him to the humans to get experimented on by them and thus breed hatred. Kei gets experimented on and tortured for 10 days. Despite being able to, he does not injure or kill those experimenting on him, as he fears that Kai would hate him if he did. He also believes that since Kai gave his one and only life to help him, if he does not give everything he has apart from his life, it wouldn't be fair to Kai.

Kei apologises to Sato

After 10 days, Satō comes back to rescue Kei, but once again realises the plan was a failure as the first thing Kei does upon being woken up is apologise to Satō for all the trouble. However, when Kei observes the bodies of people Satō killed on his way to rescuing his Kei, he says that it does not bother him, and Satō decides to observe him for a little longer as he senses Kei is a bit strange. This changes shortly after, when Kei then tells Satō not to shoot at unarmed surgeons who are not resisting, and then ends up firing at Satō, causing Satō to turn on him with murderous intent. The two engage in a battle until Kei escapes to the roof with the help of one of the scientists, who sympathised with Ajin and regretted his participation in Kei's experimentation.

On the roof, Kei asks the scientist why he decided to help Kei, and reveals to him that he has only been "pretending to care about humans" and that he is actually emotionally detached from other people and the concept of human suffering. This

Kei saves the life of the scientist

is what Kei claims, but his statements are made debatable only shortly after when he attempts to save the life of the scientist, by dropping him off the building where there is a chance of him surviving the fall, and this does end up saving his life. Kei would later in the series argue that he only did this because the scientist could be valuable in the future as an Ajin sympathiser, but his true motivation is left up to interpretation by the reader. Also on the roof, the heads of Kei and Satō's IBM briefly collide with each other, allowing them to exchange memories and mental states. Kei then jumps off the building himself and into the sea to escape.

Village arc[]

Kei and Ko's first encounter.

After drowning for a few weeks (according to Kei's approximation), he washes ashore near a small village. He meets an old woman, Yamanaka (though their initial meeting is off screen), and she allows him to live with her while knowing of his true identity as an Ajin. He poses as her grandson from Tokyo, Ryouta, under the cover story that he had been disowned by his parents after getting in trouble with the police. While living in the village, Kei goes into the woods to practice using his IBM but grows frustrated as it does not obey him, and instead does the complete opposite of what he says. After some time, Kō Nakano runs into him during one of these sessions. His IBM first kills Kō accidentally, but he then discovers that he is a fellow Ajin, and Kō asks for Kei's help in taking Satō down. Kei initially pretends to accept this offer and brings some food back for Kō, but it turns out that the food was poisoned and Kei had no intention of joining forces with him, saying that he had finally found a safe zone that he did not want to give up. Kei and Kō engage in a fight where they both try to immobilise each other, and Kei ends up winning by drowning Kō, and consequently locking him in an abandoned truck. He keeps him there for a few days while bringing him food, water, and even some magazines and a small TV for entertainment. Again during one of their conversations, Kei claims confidently that he does not value the lives of other people above his own, so he does not care how many people Satō will kill, causing Kō to become extremely infuriated. 

Kei asks Ko for help

 Despite Kei's attempts to secure a safe life for himself, it does not take very long for some of the other people in the village to become suspicious of him and suspect that he is not really Yamanaka's grandson but in fact Kei Nagai. Kei is able to barely win the trust of two of the men by giving a detailed cover story, fake-crying and bringing them alcohol as a gift. But just then, he is suddenly shot by Mr Kita, the villager who was most suspicious of him, and his identity as an Ajin is revealed. Kei runs away immediately and tells Yamanaka that he is leaving. Another man offers him a ride in his car saying that Yamanaka told him the real story and he is an ally, but then reveals that he plans to give Kei over to state custody because it is the right thing to do and the safest place for him. Kei realises the man is useless and panics, then steers the car into a tree to escape. At this moment, while overhearing the acts of terror Satō has been committing over the car radio, Kei grows frustrated that Satō is ruining the reputation of Ajin and realises that he will never be able to live in peace as an Ajin unless he puts a stop to Satō. Desperate, he frees Kō from the truck and asks for his aid. The two are about to escape but Kei receives a call from Yamanaka's phone where he discovers Mr Kita is currently threatening her. Kei tells Kō that he has forgotten something, goes back to the village, shoots Kita, tries to make the death look like a suicide, then returns to the cliffside with Kō. They jump into the water together and escape.

Alliance with Tosaki[]


Kei plans to team up with Tosaki.

ing in a car that Kō was able to secure for them, Kei suddenly announces that he is exhausted and doesn't want to stop Satō anymore - much to Kō's bewilderment. He explains that they cannot achieve anything as two teenagers on their own and need an ally. When the two are at Satō's previous protest location because Kō had come back to look for Akiyama, Kei receives a message from a detective agency he had made a request to shortly before he and Kō escaped, asking them to do a background check on Ajin Control Commission officials. He shows Kō the profile of Tosaki and says that they can use him, because Tosaki needs to pay for his fiance's hospital bills and would therefore be desperate enough to team up with them. He adds that they can also exploit his weakness, which is that he is in love, which makes Kō uncomfortable. 

The pair drive to the hospital that his fiance is being kept in a

Kei bargains with Tosaki.

nd phone Tosaki while driving, alerting him of their threat. At the hospital, standing before Tosaki's fiance, Kei tells Tosaki that they can help each other attain their goals and he and Kō can be his "immortal soldiers". Tosaki reluctantly agrees and they return to a hideout together. 

Kei and Kō begin training to get into shape and for Kei to lose some of the weight he has put on, much to the annoyance of Kei, who hates physical activity coming from a "white collar background". They are also introduced to Dr Ogura, who tells Kei that his IBM is unusually concentrated and he must have manifested one since infancy. 

Kei and Ko's argument.

Around this time, partially due to him observing Kō's ability to make friends and gain the trust of others with ease, Kei begins to question his nature and values -- being someone who has up till this point confidently believed in cold pragmatism as the most "logical" way to live by. He enters a room where he sees Manabe with his tranquilizer gun drawn (he was teaching Kō how to shoot) and uses his ajin stun scream in panic, freezing everyone in the room. This causes him and Kō to get into an argument in the bathroom. Kei tells Kō not to become so close with the other men as they are just mortal humans and can be used as pawns in battle, enraging Kō. Kei claims yet again that he has never cared for anyone in his life, not even Kaito, arguing that he had value before but now he is no longer of any use to him. He believes emotions are a nuisance, saying "Follow your heart and your body will crumble." Although, it is apparent that Kei is somewhat losing faith in his own words despite stating them so confidently.

Hirasawa reassures Kei.

While Kei is sitting alone later, he is approached by Hirasawa, who asks him if he is lonely which immediately baffles Kei. Hirasawa reassures Kei by telling him that although Kō's social skills are a useful trait, Kei's way of thinking is suited to the work they are doing, and he is fine the way he is. This causes Kei to open up a bit, and he tells Hirasawa how his father was an excellent surgeon, but because he was too emotional and cared too much for his patients, he used illegal means to buy an organ for a transplant -- causing him to lose his job and his family. Kei says that letting emotions get the better of him is a mistake he will not make, because he is "not an idiot".

Forge security arc[]

While waiting for Satō's attack in the forge security building, Kei falls asleep and has a dream where Satō attacks from a jet plane, killing his comrades. In the dream, Kaito calls Kei's phone and is standing atop a roof nearby, telling him to come with him and that they can run away properly this time, but Kei chooses to stay and fight Satō. 

A few days later, the real attack begins. Kei disguises himself as a member of forge security and plays dead when a room of other guards are open fired on by Tanaka, Gen and Takahashi. He heads downstairs with Kō to the mechanical room of the building, where he passes IBM particles through a ventilator that will create a visible-to-ajin-only smokescreen on the 15th floor of the building. Kei's plan works, and the three are tranquilised by Hirasawa. He and Kō continue to hide in the mechanical room, as they are now waiting for Satō to appear. Kei had purposefully allowed Okuyama to see him through CCTV footage and alert Satō about his presence before tranquilising him, so Satō would know that something fun was happening and would show up. Sure enough, Satō appears before them, and blows up the entire mechanical room in order to render the backup power generator inoperable, then cuts off the main power supply, shutting down all electricity in the building. Kei and Kō try to find Satō but run into Tanaka instead and have a fight with his IBM. While heading up the north stairs, Kei is recognised and stopped by the police. He pretends to use Kō as a hostage and allows himself to be hit with a tranquiliser dart, then Kō tranquilises the men who come to his aid and shoots Kei to revive him. 

Kei and Kō then catch up to Hirasawa and Satō, but the former three are all taken out and Satō escapes. Once revived by Hirasawa they head to the President's room to find he has been killed, then head for the roof. Hirasawa tells both the boys to escape and that he has another route of escaping, but Kei realises that this is a bluff and Hirasawa is already wounded and will die. When Satō appears on the roof, Kei tells Hirasawa that he will not run from the fight, but Hirasawa shoots him off the building, and is killed on the rooftop by Satō. When Kei hits the ground he is surrounded by journalists and live TV cameras, but Kō who had hijacked a broadcasting truck quickly picks him up and they drive off. 

Kei cries for the deaths of his comrades.

Kō asks Kei what the new plan is, but Kei says that he is done. He no longer thinks it is possible to beat Satō, and because he can no longer live a civilised life, he is going to run away and live somewhere remote. They end up getting into another fight where, once again, Kei tells Kō that Satō could kill thousands of people and he would still put his own life before theirs. He tells Kō that nobody really believes all lives are equal, and that people die all the time for no good reason, so this should not be any different. Kō tells Kei the story of how he first became an ajin, and says that he needs Kei and could only get this far because of him -- he cries and pleads with Kei, but Kei bluntly responds that it is not his problem -- although, it is shown that there are tears in his eyes as he walks away.

Kei goes to a telephone box and calls his mother. She speaks to him in a nagging tone and questions what he hoped to achieve by calling her, remarking that although she is his mother, there is nothing she can do for him anymore since he became an Ajin. Kei tells her that he is planning on disappearing, which his mother agrees is the right decision, and Kei puts down the phone. However, when he gets to the edge of the sea, he has thoughts of Hirasawa and his other comrades who died, and turns back, choosing to stay and fight after all.

Iruma base arc[]

Kei regains his determination.

When returning to the hideout, Kei tells Tosaki, Kō and Izumi that they currently have a 0% chance of beating Satō, but this number will never increase beyond zero unless they act and fight -- therefore, continuing to fight against Satō when they have a nonexistent chance of winning isn't stupid, but a "perfectly logical thing to do". Kei puts on a highschool uniform and they drive to Iruma City, Kei’s birth and death place, where the Ajin control commission will be having a meeting in hopes of coming to a settlement with Satō. Before they enter the Iruma City Civic Gymnasium, Dr Ogura warns Kei not to flood, as with his concentrated IBM it could turn dangerous and go out of control. Kei and Kō watch for Satō to arrive atop the building, but find that Tanaka has come alone. After observing the fake Satō that was constructed from Satō’s IBM at Iruma Base, Kei gets ready to head there. On his way, Izumi asks his permission to rescue Tanaka who has been captured, to which he accepts and asks Kō to accompany her. Before leaving, he has a brief conversation with Tosaki where he tells him that he “never once resented the fact that he became an Ajin”, as if he was not an Ajin, he would have been dead from the moment the truck had hit him, and after this, the three parties take their separate leaves. 

While Izumi and Kō are pursuing the car that Tanaka is being held in, Kei gives them directions over the phone. He then arrives at the wall around the base, and climbs over it by biting off his fingers, placing them against the wall, slicing his neck with a shard of glass, and repeating this process till he reaches the top. Once inside, he watches Satō’s live broadcast and although he does not have a proper plan, announces that he will try and get close to Satō. He runs onto the base runway realising that Satō will come outside to get into a fighter jet, and he is spotted by cameras and appears on live TV. He explains his new plan to the others to get hold of Satō’s severed arm and drop it through a pipe or hole into the ground, having him revive into a sealed environment. While doing so, he is sighted by a few of the twelve humans who Okuyama recruited who close in on him, but is then rescued by the anti-demis. He tells the anti-demis to go to one of the marker’s for Satō’s arm by themselves, and goes back by himself to the runway to execute a facade plan of lining cars up on the runway to block Satō’s path, so that Satō will not realise their real plan of searching for his arm. He tells Kō that they will need to find a hole in the ground to drop the arm through in roughly 5 minutes. Once the hole is found, he yells through the phone to Kō that he knew he could count on him. However, although they were able to drop the arm in the hole on time, the plan failed as Satō simply got bored and did not blow himself up again. 

Kei’s expression at this point becomes extremely exhausted and deflated. While he is sitting on the runway, Satō flies towards him in his fighter jet. He then approaches him and says that he has come to say a final goodbye as he has gotten bored of Japan and will now be going overseas. Completely unexpected to both of them, Kaito appears riding a motorbike and catches Satō off guard. Kei uses the opportunity to fire his tranquiliser gun at Satō, but as soon as Satō is hit, he detonates the explosives that were strapped to him and revives elsewhere (as he had realised earlier of Keis’ true plan, and had already shot himself and severed a new arm, not the one Kei’s team dropped into the hole). 

Kei has a reun

Kei and Kaito are shot by Sato.

ion with Kaito, where he asks him what he is doing and why he came to such a dangerous place when he told him to stay away, and that he should run and find shelter immediately. Kaito acknowledges that it is a dangerous place for someone mortal to be, and states that he hates how much of an idiot he is -- but Kei remarks that he is the biggest idiot of all, expressing his frustration of becoming too confident in his plan that ended up falling through. The conversation does not last long as Satō reappears behind them and fires a bullet that shoots through the back of Kaito’s neck and through Kei’s eye sockets to his brain.

Kei gives Kaito CPR.

The intense mental state that Kei enters due to Kaito being killed activates a flood as Dr Ogura had warned of, and hundreds of IBMs are created. While Satō’s attention is occupied with this, Kei notices that Kaito’s bullet wound has healed, and drags him underground to try and revive him. Kaito has no pulse, but he furiously tries to give him CPR while thinking of how Kaito has always been so selfless and helpful to him, while never giving Kei a chance to repay the deed. Kaito’s heartbeat and gentle breathing seems to be restored, and Kei climbs back up to the surface to face Satō one last time. 

Satō is about to leave the country by a helicopter that his IBM is driving, but Kei jumps up and holds onto Satō’s leg at the last second. He tries to tranquilize him a

An amnestic Kei realises he is an Ajin.

gain but fails and is dropped from the sky. Satō also falls, as he is shot by Manabe who sniped him from a nearby roof. It turns out that Kei’s fall did not kill him, but hitting his head gave him amnesia. He does not recognise Satō and cannot remember anything that happened in the past two months since he was first hit by the truck. Upon finding Hirasawa’s gun in his bag and tying events together, he concludes that he must be an Ajin and shoots himself in the neck to revive and lose his amnesia, and all his memories come flooding back (ironically, it is at this point that his flood of IBMs dissipate at once).

Kei starts running to catch up with Satō’s helicopter.



Kei used to be good friends with Kaito when they were very young, though they did not attend the same elementary school. However, when Kei's mother told him that Kaito was the son of a convict, he cut off connection with him without hesitation and they did not speak to each other ever since. However, Kaito is the only person Kei can think to call when he first becomes an Ajin, and Kaito helps him without second thought. While on the run together, Kaito defends him physically a number of times and tries to calm him down verbally too. Eventually, Kei feels bad for getting Kaito involved in his own dangerous situation and "abandons" him, for his own safety. While undergoing torture, he does not harm those experimenting on him because he fears Kai would hate him. Kei also dreams of Kaito rescuing him in a dream at one point. Despite all of this, Kei (wanting to believe he is apathetic and unemotional) dismisses that he cares about Kaito when arguing with Ko, saying that he only valued him when he was of use to him, but that does not apply anymore. This is clearly not true, as it does not add up with all of Kei's other interactions with Kaito. Furthermore, when Kaito breaks out of prison to rescue Kei later and ends up getting himself shot by Sato, Kei desperately tries to revive him, expressing frustration that he was never given a chance to repay Kaito's kindness. It is later revealed that Kaito initially did not help Kei because of any special personal feelings towards him, but just because it is his nature to help anybody who is in trouble, and he believed that Kei would be "hopeless on his own". However, after Kei demonstrated care for his wellbeing and safety upon "abandoning" him, Kaito has a new perception of Kei - as a real friend who pays back his kindness - and it seems he is more enthusiastic to rescue Kei after this for more personal reasons.

Kō Nakano[]

Kō and Nagai had a rough start. Nagai was unwilling to accept Kō's help, leading it to several fights and ending up with Nagai locking Kō in an abandoned truck for several days. However, after eventually getting himself into more trouble, Nagai frees Nakano from the truck and asks him for help. Kō seems to hold a grudge against him, but still trusts and looks up to Nagai, he agrees to do so anyway. They seem to get along normally after some time, but still bicker around often. Ko ends up seeing Nagai as a close friend, and Nagai slightly warms up to him over time- keeping his sarcastic edge though. Kō seemed to change his mind about Nagai, saying he actually does care about people, which Kei immediately denies.

Yū Tosaki[]

Their relationship is quite ironic, as they are technically on opposing sides (with Kei being an Ajin and Yū a human) but decided to work with each other notwithstanding. Kei only did it as opposing Satō would only put himself into danger, and he needed a strong ally. Tosaki was reluctant at first, but eventually agreed as it was the most logical thing to do. On casual occasions, Kei and Yū bicker around with a lot of sarcastic comments.
Tosaki also forces Nagai to train vigorously, which Nagai hates. Ironically, though, their personalities are not so different as it may seem as they are both very cold and analytical. However, Kei considers Tosaki's love for his comatose fiancee a weakness and something that he can use against him.
It is shown that Tosaki holds respect for Kei's intelligence and occasionally asks for his opinion on matters he is skeptical about and usually asks for his advice when planning missions or when their plans are compromised.

Izumi Shimomura(Yōko Tainaka)[]

They have not interacted much, but they seem to have a mutual understanding of each other as both of them are Ajin. Kei also seemed to wonder how easily Nakano was able to make friends with her.

Ritsu Nagai[]

Kei's mother, whose cold and analytical demeanour he inherited. Her understanding of his nature helps her guide and influence Kei in his path to becoming a doctor like she and her husband were. Her opinion is important to him, as when she told him to stop being friends with the son of a criminal, he immediately obeyed as to not get his reputation ruined.
Despite the seemingly heartless nature his sister sees in him for this abandonment, his mother reveals his true nature to his sister in chapter 44. Due to his analytical nature, if he comes between choosing between two choices, he chooses what he considers to be more important. When the choice between continuing to be Kai's friend would jeopardize his chances to save his sister, he prioritized his sister over his friend.
However, despite taking so much after her, she knows he inherited his father's dogged nature. Therefore, instead of running to sea and away from the terrorist actions of Sato and the government who would imprison him, he will fight Sato for those who have died.
In the anime, their relationship is slightly different, adding an extra scene where she asks Kei about his grades and that she doesn't expect anything bad. Kei tells her not to worry.

Eriko Nagai[]

Eriko is Kei's younger sister. She is hospitalized and Kei visited her in the hospital on several occasions. When they were younger, she asked Kei why did their puppy have to die and she was upset when they called the dog just a piece of defective merchandise. Kei tells her that's just what happens with sick things. She wonders if all people die when they are "defective".
Even earlier on, she seemed to notice that Kei was doing everything for his own benefit rather than others. She holds resentment to him for being selfish and detached. When questioned by authorities, she was unsympathetic for him when she discovered that her brother is an Ajin. Although later on in the manga, it is shown that she does hold some sort of affection for him.
Kei doesn't hold close regard to his sister and barely, if ever, thinks about her well-being later on in the manga. In the anime, however, he immediately runs to her rescue when he learned about the fact that she was being targeted for being his sister. It is revealed in chapter 44 that his reason for wanting to be a doctor is so he can find a way to cure his sister and save her life.


At first, Satō seemed almost friendly to Kei to earn his trust. He kept this façade (to Tanaka's distaste) in order for Kei to think that he was on his side. That's why he made the decision to shoot Kei with a tranquilizer gun (making Tanaka use his IBM) to make it look like the humans where attacking, instead of simply kidnapping him, stating that "it would just make him hate us". Kei not being aware of this, wakes up strapped up to an operation table, cue to his ten day torture.
Later on, Satō tells him that he came to save him. Kei apologizes for the trouble he has caused. While they were "escaping", Kei tells Satō that he doesn't care about the lives of other people, but that it's unnecessary to kill people that aren't even involved. Satō decides to drop his façade and opts to kill Kei instead. Kei manages to escape with the help of one of the doctors, who was later stated ended up in a coma.
As the story progresses, Kei (along with Kō Nakano) has decided to join forces with Tosaki and help him take down Satō, not because he has caused multiple terrorist attacks, but because too much attention would be brought to Kei, therefore putting his own life in danger.

Abilities and Powers[]

  • Immortality –  Like all Ajins, Nagai is immortal, he is unable to die under any circumstances such as being completely crushed, decapitation, sickness, or even due to malnutrition. However, all ajin die when they reach the end of the average human lifespan- due to complications of old age. Nagai fears that if his head is decapitated and a new head grows on his body, he would have no control over his body anymore.
  • Paralysis Scream – When Kei screams he can paralyze the people around him, it's been stated to have less of an effect if the person is unaware the user is an Ajin or if they are close to the ajin.
  • IBM / Black Ghost – Like all Ajin, Kei possesses IBM however he has an abnormal amount of it and thus he can manifest more than the average Ajin. Kei can also manifest a Black Ghost and unlike Ghosts of other Ajin, it is considerably plain without any distinct deformity. Kei's IBM/Black Ghost is a unique case since it was with him since childbirth, where he had first died. Even though it first appeared to him when he was a child, he is unaware of it. Years of lack of association between him and his Ghost has lead to it being extremely disobedient and self-propelled, acting independently with a mind of its own nearly killing him once on purpose and seems to dislike Kō Nakano for unknown reason and every time he sees Nakano, he attempts to kill him. However, it's been shown in both the manga and the anime that he had been studying and training his IBM/Black ghost and could, in some level, control it. It is known that he can summon multiple ghosts and each Ghost can last for as long as 30 minutes. He has stated that "on a good day" he can summon 9 ghosts but normally 5, a result of his high IBM concentration.
    • In the manga, Ikuya Ogura believes that if Kei experiences a flood phenomenon, it would be a significant amount more IBMs summoned compared to other Ajins due to his IBMs being 3 times denser than the average Ajin.
    • In the finale of the anime: Ajin 2, Kei creates many IBMs and experiences the "Flood" phenomenon.


  • “Kai...for my sake...he waged his life...his one and only life...for someone like me. I could never do something like wager my own life, therefore….if I don’t risk everything I have besides my life, it won’t be fair to Kai.” (chapter 7)
  • “I haven’t been concerned for anyone, I’ve never felt the need to. I couldn’t have cared less about others! And even now I...don’t give a damn about them! But I want to save you!” (to scientist, chapter 12)
  • “He’s in love. That weakness gives us an opening. We’re teaming up with him.” (to Ko, about Tosaki, chapter 23)
  • “It just gets more and more clear to me. spare emotions just make bad situations worse. Clinging to them won’t get you out of trouble. Honestly, I’ve known for a long time now. Let your heart guide you and your body will crumble.” (to Ko, chapter 25)
  • “People don’t change.” (to Ogura, chapter 26)
  • “When i speak, it always makes sense. I’m delivering rational decisions. give me one reason why I’d need to change.” (to Hirasawa, chapter 26)
  • “I’m just moving my pieces into the most favorable position for me. Why would I do anything else?” (to Ko, chapter 34)
  • “All of us weigh the lives of others on a scale in the back of our minds. Don’t go around criticising me just because I do it consciously!” (to Ko, chapter 43)
  • “We have a 0% chance of winning right now. That number will stay at zero forever unless we intervene. So even if it comes to nothing, we have to keep acting. It can be a 0.00001% chance. We have to do this to keep the possibility alive of creating a way to win. Do you understand what I’m trying to say here, you idiot? Fighting to the end, even if it comes to nothing, isn’t an illogical act! This is a carefully considered way of maximising what is our current 0% chance of winning! It is a perfectly logical thing to do!!”  (to Ko, chapter 48)
  • “You talked about friendship so much that it came off as fake...but maybe you knew better than anyone. The two of us aren’t friends. If someone’s in trouble...it’s just what you do. But this time...you’ve gone too far! Come back! Give me at least one chance...to do something for you.” (to Kai, in thoughts, chapter 73)


  • 永井 Nagai means eternal well, while 圭 Kei means edge.
  • His seiyuu, Mamoru Miyano, sings the ending theme in the first season of the anime.
  • His English voice actor, Johnny Yong Bosch, also voices Persona 4's Yu Narukami. Like Nagai, Yu Narukami is a teenager who is exceptionally skilled at summoning a guardian spirit.
  • Nagai can be seen wearing a shirt that says "All men by nature desire knowledge." in season 2.
  • Nagai isn't very good at cooking, and likes to eat junk food a lot of the time
  • Nagai is shown to be quite neurodivergent-coded

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