Nagai was first shown with an academic, serious, almost cold personality. He would often get lost in his thoughts, leading him into getting in a car accident as well having his identity as an Ajin revealed to the entirety of Japan. Afterwards, it is revealed that Kei is a complete pragmatist and emotionally detached from others. He is very intelligent and hardworking with ambitions to be a doctor who finds treatments or cures illnesses that don't yet have one