Kei Nagai is the protagonist of the anime series 'Ajin.' He is a high school student who discovers he is an Ajin, a type of immortal being, after surviving a fatal accident. This revelation thrusts him into a world filled with danger and intrigue, as Ajin are hunted by the government for experimentation. Kei is intelligent and resourceful, often relying on his sharp mind to navigate the challenges he faces. His discovery of his Ajin nature dramatically changes his life, forcing him to confront issues of identity and morality.