Akuma

He's Satan, the strongest demon in hell. He was once an angel but was cast away to hell by god due to some reason. He appears before Miu and says that he would grant a wish but in return for granting that wish she has to give her virginity to him according to the contract. But as the story progress he falls in love with her and even goes as far as to tell Miu that even though he will let Miu have Amamiya's heart, he would not let Amamiya have Miu's heart because it belongs to him. He takes a few roles in Miu's life. First, he became Miu's elder brother, Kai. After they confess their love to each other, he becomes Miu's biology teacher. He is not very fond of Amamiya and refers to him as "that fucking angel". (Source: Wikipedia)