Kei Nagai

Kei Nagai

Kei anime

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

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


Kei at the start of the manga.

Kei at the start of the manga.

Kei is a young man with a thin stature, disliking 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. Like most Ajin in the anime, his eyes are a dark red colour. At the start of the manga, his eyes appeared black.
After undergoing several dissections and experiments, his hair is seemingly shorter and spikier. Later on it grows back to its original length. In the anime, it stays the same length.
Kei usually wears a white shirt but his top has been shown to change to a normal shirt in recent events. He accompanies his shirt with plain black trousers. When they were chasing down Satō, Kei (as well as ) disguised himself as a bodyguard, wearing a shirt with black stripes on the shoulders, a black tie, jeans, and a cap to conceal his face.


Kei was first shown with an academic, serious 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 cures illnesses that don't yet have a cure; specifically to cure his sister Eriko. However, Eriko observes that Kei is not thinking about her at all and is only pursuing this goal for his self-interest.
Despite Kei's lack of emotional investment in others, he 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 also is 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.
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 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, she 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 due to Perinatal Hypoxia but was revived due to his status as an Ajin. The doctors who were trying to treat him believed it was a miracle.
When Kei was younger, he saw the black ghost. It happened when he buried his dog with his little sister. Kei 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. Kei's mother later explains that cutting off Kaito was necessary to become a doctor and save Eriko.
After he is revealed to be an Ajin, he goes on the run with Kaito. As Kaito shows unwavering loyalty to Kei, Kei begins to feel guilty about his mistreatment of him. When Kaito gets hurt, Kei leaves him and begin the journey himself, pushing himself to the point he almost dies, when Satō and Tanaka who has kidnapped his sister. 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. Afterwards, Kei gets experimented on and tortured for 10 days, with thoughts of Kaito to keep him going.



Kaito stated that Kei was his first ever friend. He was the person that Kei called in his time of need and has been helping him in his struggles ever since. It is shown that Nagai felt guilty about leaving Kaito and stealing his mobile phone, however its shown that Nagai somewhat cares for him because of the fact he left clean water with him instead of taking it for himself.

Kō NakanoEdit

Kō and Kei had a rough start. Kei was unwilling to accept Kō's help, leading it to several fights and ending up with Kei locking Kō in an abandoned truck for several days. However, after eventually getting himself into more trouble, Kei frees Nakano from the truck and asks him for help. Kō seems to hold a grudge against him, but still looking up to Kei, he agrees to do so anyway. They seem to get along normally after some time, but still bicker around often. Kō seemed to change his mind about Kei, saying his actually does care about everyone, which Kei immediately denies.

Yū Tosaki Edit

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) Edit

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 Edit

Kei's mother, whose cold and analytical demeanor 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 Edit

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.

Satō Edit

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 PowersEdit

  • Immortality –  Like all Ajins, Kei is completely immortal, he is unable to die under any circumstances such as being completely crushed, decapitated, or even due to malnutrition. However, he 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 aware 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 IBM's and experiences the "Flood" phenomenon.


  • 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 Kei, Yu Narukami is a teenager who is exceptionally skilled at summoning a guardian spirit.
  • Kei can be seen wearing a shirt that says "All men by nature desire knowledge." in season 2.

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