|First Appearance||Chapter 8|
Keisuke Nakajima (中島 啓介 Nakajima Keisuke) was a student from Nagai Kei's high school.
Keisuke has very short, black hair. He has thin, sharp eyebrows and medium colored eyes. He was seen wearing a dark t-shirt.
Keisuke is a surprisingly kind boy and he feels sorry for Nagai after he discovers about the Ajin case. He searches for Ajin ally groups on the internet, stating that he thinks Kei, as well as other Ajins don't deserve this and that it's cruel that they have their lives ruined just because they're Ajins.
He is shown not to be very academic, saying that he doesn't care about studying. He is confused and upset when his former girlfriend, Yuki, dumps him over this.
Keisuke is also shown to get flustered easily, given his reaction to being teased and also by seeing Kei's notes.
He lives in the Saitama Prefecture. It is known that he attended the same cram school as Kei for college entrance exams. He, along with his former girlfriend, planned on attending the same college after they are done with mock examinations. He promises to Yuki to meet up with her and study for the exams. However, Keisuke becomes too distracted after the Ajin incident and does poorly on his exams. He also forgets about his meetings with Yuki, and she eventually dumps him.
He was seen at the summer training session near Iruma Station of the Seibu Ikebukuro Line.
He, along with Watanabe, have witnessed the accident in which Kei gets run over by a truck and later comes back to life.
- The database lists his postal home number as 358-0011, with the description of "Saitama Prefecture Iruma Shimofujisawa" as the address. However, there is no description of this address associated with such a postal code in real life. The nearest station is at Seibu Ikebukuro-sen Musashi Fujisawa Station.
- The kanji in his given name mean "opening" (啓) and "mediate" (介). The latter kanji is the same kanji that is used in Yūsuke's name (祐介). His surname, Nakajima, means "island in a river".
- He is the fifth person whose surname contains the kanji 中, the other four being Nakano Kō, Tanaka Kōji, Tainaka Yōko and Nakamura Shinya, as well as the third person whose initials are K.N (Kei Nagai and Kō Nakano).