Diabolik Lovers: The Hidden Vampire Princess
The Sakamaki brothers have a younger sister who's precious to them and she's in a deep sleep, her name is Sakamaki Airi, a 15 years old girl and the daughter of Karlheinz and the deceased Elaine (the 4th wife of Karlheinz).
The only one who could wake her is Karlheinz but there's a prize that must be paid. The Sakamaki brothers must obey and follow Karlheinz's Adam and Eve project.
The plot's originally mine, so no copying!!!