In the 21st century, a family of vampires settled down on the grounds of Japan and lived a modest life whilst sucking on the blood of humans discreetly. However, the eldest daughter of the family, Karin Maaka, has an embarassing secret which cannot be revealed to anyone. Even though she is a vampire, she does not lack blood like her family members. Instead, she overproduces blood to the extent that she transfuses the excess into her victims, hence increasing their blood levels. Despite her troubling body characteristic, she attends school like any other high school student, that is until a new transfer student, Kenta Usui, enters her life.
Karin Maaka (真紅 果林, Maaka Karin
?) is descended from a long line of purebred vampires. However, instead of having to drink blood like normal vampires, Karin's body overproduces blood. Eventually, she reaches a point where she can no longer hold it, and she must bite someone to inject them with her blood, or she suffers from a massive nosebleed. If Karin goes too long without biting someone and tries to hold her blood in, she goes into a "berserker"-like state and aggressively attacks the first target she can find. Due to her unusual state, Karin lives like a normal human, rather than a vampire, living in the daylight and lacking the skills normal vampires have. This has resulted in her being somewhat separate from her family, who she only sees for a few hours in the time before she goes to sleep, except for Anju who has not yet awakened as a vampire. As the series progresses, Karin falls in love with Kenta Usui. He returns her feelings, though Karin is initially troubled to learn that human-vampire children are born sterile.
After Karin is rendered unconscious for days after she tried to hold in her blood, her family grows increasing concerned that her condition will kill her. Her mother journeys to her birthplace in search of answers and learns that Karin is a rare vampire, called the "Spring of Psyche", that is born when the vampire species is struggling and that the blood she produces can bestow fertility in vampires. Karin is later kidnapped by other vampires who want to feed from her to rejuvenate their own lives, though it will kill Karin in the process. They also planned to impregnate Karin against her will to continue the Psyche's existence. After Kenta and the Maaka family came to save Karin, her nose-bleeding ceases, and she becomes a normal human, who will likely grow old and die. Her family erases all of her memories of vampires, including themselves, promising Kenta that they will continue watching over her happiness though she will not know them. At the end of the series, Karin and Kenta have married and are shown to have a teenage daughter named Kanon.
In the anime adaptation, the blood-making issue and Karin's capture are not mentioned. Instead, Karin has the opportunity to become a full vampire, which she initially declines, but then pursues after Kenta encourages her. The transformation is interrupted and Karin goes berserk. Kenta comes to her rescue by volunteering to be bitten, ending the berserker rage, confirming their romantic relationship and simultaneously resolving Karin's issues with being a blood-maker. In the anime television series, Karin Maaka is voiced by Sayuri Yahagi in Japanese and Chelsea Curto in English.