Day One is the 25th chapter Ajin: Demi-Human, created by Gamon Sakurai.
Kei and Nakano undergo physical training while under Hirasawa's supervision while Tosaki tries to devise a plan to catch and hold Sato as quickly as possible and tells Izumi that this would be their outpost.
Later Izumi comes up to Kei and Nakano and tells them to follow her and that they need to meet someone. On the way Nakano whispers to Kei about how pretty Izumi is and wonders if she and Tosaki are a couple. Kei points out to Nakano that since she was the only one at the hospital that wasn't wearing earplugs.
Once Kei and Nakano meet Ikuya Ogura, Kei gets asked to bring out his IBM so that Ogura can see it. When Kei tries to give his IBM an order it completely ignores him and kills Nakano instead. Ikuya compares Kei's IBM to a case in Iowa and points out that Kei must have left the IBM for a long time. Kei's IBM tries attacking Ogura but Izumi's IBM quickly stops it. Ogura tells Izumi to leave it alone as it will disappear in 5-10 minutes but Kei tells Ogura that his IBM lasts longer and that he can bring out 5-9 IBM per day. Ogura comments in amazement that Kei's IBM is unusually concentrated.
Afterwards due to a misunderstanding, Hirasawa's group is struck by Kei's scream. When confronted by Nakano, Kei tells him not to be so friendly with them and that they are just sacrificial pawns. Shocked by Kei's overly harsh nature, Nakano asks Kei whether there has even been that he cared for. Kei tells him no and that emotions only ever get in the way.