As a result of abuse and mistreatment by humans in the past,Nezu's true character reveals itself whenever he is in combat.He enjoys"toying"with humans.He still holds a grudge over the many experiments he went through,coming off as slightly insane and unpredictable as a resultAs befitting to his role as principal of an academy for heroes,Nezu is an eccentric, polite,and persistent educator.He is quite self-aware of his odd appearance and introduces himself by pointing it out.He isn't a human.