Origin
Japan
Eriko Nagai (永井 慧理子, Nagai Eriko) is the younger sister of Kei Nagai. She is a human. Due to not having much debut in the manga, not much is known about her except that she holds resentment towards her brother for not becoming a doctor to treat other people but rather for his own sake. She claims that Ajin's shouldn't be treated as humans. Otherwise, Eriko seems to care about those close to her, and seemed the most sympathetic out of the Nagai family. Her brother. 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 defected merchandise. Kei tells her that's just what happens with sick things. She wonders if all people die when they are "defected". 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 holds no sympathy for him when she discovered that her brother is an Ajin. She thinks that they deserve severe punishment. Kei doesn't hold close regard to his sister and barely, if ever, thinks about her well-being later on in the story.
Ajin
Ajin are humans that cannot die. Seventeen years ago, they first appeared on a battlefield in Africa. From then on, more of their kind are discovered within the human society. Their rarity in appearance means that, for experimental purposes, the government will generously reward anyone who captures one. In the present day, for what a certain high school student expects to be a typical summer holiday, his life is about to turn unexpected...
Ajin 2nd Season
After escaping certain death, Kei Nagai and his new companion Kou Nakano plot revenge on Satou, their fellow Ajin who is hellbent on world domination. As Satou embarks on a string of public executions, the human race rushes to come up with a solution to stop the immortal villain. Kei discovers unlikely allies in the form of two former adversaries: high-ranking government official Yuu Tosaki, whose extensive research on Ajin gives him a tactical advantage in the fight against Satou, and Tosaki's Ajin assistant Izumi Shimomura. As his faction continues to gather allies, Kei races against time to put a stop to Satou's crusade before it brings about an end to civilization as he knows it.
