Ma Furu Yoru no Rin

Akito, a normal but perverted student accidentally got dragged into a fight between demons and a beautiful demon hunter, Rin. During the fight, Akito tried to protect Rin from a fatal attack and became infected with a demonic virus. He was destined to become another demon, unless he survives 7 days without an excessive accumulation of negative emotions and abstains from using his demonic powers as much as possible. To help Akito, Rin decided to watch over him for 7 days, and if he fails to keep his humanity in check, Rin will kill him herself. Not wanting to get killed, Akito decided to seduce Rin and make her his woman.