Kei Nagai is an Ajin and the main protagonist of the series.

Kei was first shown with an academic, serious personality. He is very intelligent and hardworking with ambitions to be a doctor who cures illnesses that don't yet have a cure; specifically to cure his sister Eriko.

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 also doesn't begrudge Satou for having killed others to free him, but does not support killing more people unnecessarily.

(Source: Ajin Wiki)