Lunaris Filia ~Kiss to Keiyaku to Shinku no Hitomi~

Sousuke holds two precious childhood memories: one he must never forget, and one he'll always cherish—his encounters with Silvia, the vampire princess he's forgotten, and Yukari, his ever-present childhood friend. Years later, Silvia returns to fulfill their blood pact, only to find Sousuke has no recollection of her—and worse, his blood has been "tainted" by another vampire. To vampires, a blood pact is like a sacred first love; stealing someone else's contracted partner is taboo. The culprit, someone very close to Sousuke, is blissfully unaware of her transgression. Before Silvia can forcibly break the pact, Sousuke's father proposes: since the pact relies on mutual affection, whoever Sousuke loves most by the next lunar month will keep their contract. Not content with this, Sousuke's adopted sister Mai declares she'll join the "game" to protect her beloved brother. Sousuke's peaceful days are over…