Lunaris Filia ~Kiss to Keiyaku to Shinku no Hitomi~

Two important childhood memories: one that mustn’t be forgotten, and one he’ll never forget. These are the memories of Sousuke’s childhood encounters with two important women in his life: the now-forgotten vampire princess Silvia, and his always-close-by childhood friend Yukari.

Now, many years later, Silvia returns as promised to fulfill her blood pact with Sousuke, but things didn’t go as planned. Not only does Sousuke not remember her, but his blood has been “tainted” by another vampire without him knowing it. To a vampire, the relationship with their contracted partner is a lot like a “first love”, and it’s totally forbidden and taboo for another vampire to “steal” someone under another vampire’s pact. Unfortunately, the vampire responsible — someone very close to Sousuke — doesn’t appear to be aware of what she did.

Before Sylvia can forcibly undo the improper blood pact to restore her own, Sousuke’s father proposes a special arrangement: since the blood pact depends on the mutual feelings of the contractor and contracted, within the next lunar month whoever Sousuke comes to love most will keep their contract. Not happy with the way things are going, Sousuke’s (adopted) younger sister Mai declares suddenly that she’s entering the “game” as well in order to protect her beloved older brother from these vampires. Sousuke’s peaceful days are over…

[From Relentlessflame]