Keraku no Ou

No friends, daily beatings by bullies and even his parents have given up on Ryuuichi. However, his life changes one day when a new girl transfers into his class. Chihiro is an old childhood friend, and she remembers Ryuuichi. She is unaware that he has suffered brain damage from an injury he suffered while they were playing as children. Riddled with guilt, she offers to do anything for him. Taking advantage of the situation, Ryuuichi demands sex from her. Her conscience does not allow Chihiro to back out, and she agrees. Ryuuichi quickly learns that he is infallible in bed, that he has a secret weapon. After years of abuse at the hands of his classmates, he only has one purpose... Revenge. He sets out on a carnal, perverse journey to turn all the women that disdained him into his personal sex slaves...

[from AniDB]