Kei Nagai is the main protagonist of the series Ajin. He is an Ajin and lived a normal life with a bright future until he got into a car accident and was discovered to be an Ajin. He is the anti-hero of the series, who has greedy and selfish desires and uses whoever is necessary to succeed his plans.
Kei was first shown with a carefree personality. He often day dreams which led to him getting in a car accident and having his identity as an Ajin revealed to the entirety of Japan. Afterwards, it is revealed that Kei is overly logical 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. However, his sick younger sister 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 is also 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 doesn't support killing more people unnecessarily. He is ultimately pragmatic, calculating, commonsensical, manipulative, and utilitarian though, as he saves a doctor's life who helped Kei escape out of his values, but also out of utility as he sympathized with Ajin and could be a satisfactory delegator to the government. He is shown to be unafraid to die as he will just come back to life but he is cautious of the consequences of dying.
According to his sister, Eriko, Kei is logical, heartless, and perspicuous to the core and is only appearing to care because the situation demands him so.
Kei is a young man with normal stature with short black hair that stems off in separate parts, which fall in front of his forehead. He has thin black eyebrows that fall above his eyes. Kei has crimson eyes which often appear black. After undergoing several dissections and experiments, his hair is seemingly shorter and spikier. Later on it grows back to its original 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 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.
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.