Setsuna: Vampire's Love of 100 Days.

A vampire, Brado, becomes responsible for taking care of a girl, Setsuna, in exchange for his life saved by the evil that created her. The evil tells him that if she doesn't have sex for a hundred days, she'll become a full-fledged vampire, and Brado will be able to gain complete control of her. However, if he breaks the promise, both of them will die, so he reluctantly takes on the care of her. While he sleeps, Setsuna begins going to nearby town and gradually learns how precious human lives are through her observations. When she meets a young priest in a church, her affection to humans becomes stronger-- and trouble begins to brew when Brado realizes that she is falling for the priest.