She is a 17-year-old student who lives with her father, who owns a church. She was forced to live in a mansion with 6 mysterious people who, much to her shock, were actually vampires. At first she didn't believe they were real but came to accept them later on. In a struggle for survival she decides to live with them, while bearing the fear of being attacked by each of them. As time progresses, she learns a lot of things about herself that she didn't think she had. Yui's personality varies by the choices the player makes.In the first episode of the anime adaption it is revealed that she is adopted but is later also the vessel of the triplets' mother, Cordelia, whose heart is inside Yui's body, which is why everyone becomes attracted to drinking Yui's blood.