He's Satan, the most powerful demon in hell. Once an angel, he was banished to hell by God for reasons unknown. He appears before Miu and offers to grant her a wish, but according to the contract, she must give up her virginity to him in return. As the story unfolds, he falls deeply in love with her and even declares that although he will allow Miu to have Amamiya's heart, he won't let Amamiya have Miu's heart because it belongs to him.
He takes on multiple roles in Miu's life. Initially, he becomes her elder brother, Kai. After they confess their love for each other, he transforms into her biology teacher. He holds a strong disdain for Amamiya, referring to him as "that fucking angel."
(Source: Wikipedia)
