|First Appearance||Chapter 1 (Manga)|
Ajin Part 1: Shōdō (Film)
Episode 1 (TV Anime)
Eriko Nagai (永井 慧理子, Nagai Eriko) is the younger sister of Kei Nagai. She is a human.
- Eriko is a young girl of rather short stature. Eriko has long, dark brown hair split into two braids and bangs. She has dark brown eyes and thick eyebrows. When she was younger, instead of braids she had two high ponytails, and still had the same bangs.
- 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.
- Eriko seems to suffer from sort of terminal illness that was never revealed in the manga. She is hospitalized throughout most of the series.
- 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.
- Her mother. Eriko views both her mother and brother to be too cold and rational, and seems to be hurt by her words when she talks about the dog as it was something materialistic and easily replaceable; a mere deficient item, rather than a living being. Her mother seems to somewhat neglect both of her children, showing little care about them; both when Kei ran away, and with Eriko's illness. It is yet to be revealed what kind of relationship they exactly have.
- Her surname, when using the kanji 永井, means (ironically) "eternal well".
- Her illness was never revealed for unknown reasons.