J, whose full name is John Doe, is a mysterious and enigmatic demon in the manga series MADK. He is the primary antagonist and plays a crucial role in the protagonist Makoto’s descent into the underworld. J is characterized by his seductive charm, manipulative personality, and ambiguous motivations. He guides and tempts Makoto, challenging his morality and humanity. J’s presence is central to the story’s exploration of taboo desires, transformation, and the boundaries between human and demon.