Keiji Mogami is "This Century's Greatest Psychic" and a former Psychic who turned into an evil spirit upon death.

Keiji Mogami is a very bitter man, sick of the greed and selfishness of humanity. Because of the private dealings he did while alive, he has lost his moral compass and has become a twisted variant of his former self. After becoming an evil spirit, he vowed to use his power for his own intentions and fix the world by possessing people who appear to gather grudges around them. To that end, Mogami is willing to destroy a girl's body and brutally impale her father to achieve his whims.

(Source: Mob Psycho 100 Wiki)