Karin Maaka is descended from a long line of purebred vampires. However, instead of having to drink blood like normal vampires, Karin's body overproduces blood. Generally, once a month she reaches a point where she can no longer hold it in, and she must bite someone to inject them with her blood or else suffer from a massive nosebleed. In the anime, if Karin goes too long without biting someone and tries to hold her blood in, she enters a "berserker"-like state and aggressively attacks the first target she can find. Being near a person suffering misfortune causes Karin's blood to increase ahead of schedule. Due to her unusual situation, Karin lives like a normal human rather than a vampire. She lives in the daylight, is not susceptible to common vampire weaknesses. and lacks the skills normal vampires have. These differences cause her to be somewhat separate from her parents and older brother, whom she only sees for a few hours in the time before she goes to sleep (she spends more time with her younger sister Anju, who has not yet awakened as a vampire). In the first chapter Karin meets Kenta Usui and is dismayed that her blood increases around him. He has a lot of misfortune in his life. Karin resolves to do favors for him and thus reduce his misfortune and her blood problems around him. As the series progresses, Karin falls in love with Kenta. He returns her feelings, though Karin is initially troubled to learn that human-vampire children are born sterile.