Yui was an abandoned child who was later found by Richter. He transplanted Cordelia's heart into her and gave her to Seiji. She then became the adoptive daughter of priest and famous vampire hunter Seiji Komori. Later, it's revealed that the Vampire King Karlheinz chose her to be "Eve".
Yui is a sweet and kindhearted girl who is very innocent and naïve due to her upbringing by her adoptive father. Throughout the series, she proves to be very still willed and brave when dealing with the boys who usually threaten often refuses to submit to their will. However, Yui is still human and is afraid of them when they threaten her. Still, Yui tries not to bare any ill will or feelings towards her captors.
Despite her innocence and meekness, Yui can be very stubborn when the boys push her too far. She's well known in the series to be very religious and deeply believes in God. In fact, she's very devoted to her religion and whenever the boys mock her for this she remains very strong. Yui also has a deep love for her father which remains unchanged when she descovers he hid the fact that she was adopted from her. Even though in certain endings Seiji tries to kill her she still sees him as her father and seems to have no interest in looking for her biological parents.
After Laito pushed Cordelia from the balcony, Richter, the Sakamaki brothers' uncle and Cordelia's secret lover, found her dying. She asked him to cut out her heart and implant it into another body. He did as she asked and implanting it into Yui, a then abandoned infant, who had the ideal body for Cordelia's heart.
He then handed Yui to vampire hunter Seiji Komori, to avoid "Cordelia" from being taken away by his brother Karlheinz. Seiji, who was also a priest, owned a church and raised Yui since then as his daughter. Yui had been told that her mother had died when she was a child.
Because Cordelia was the daughter of the Demon King, her blood had a power flowing within it. Since her heart is inside of Yui, Yui is a power blood bank for the vampires. All the brothers note that Yui's blood is of the "finest quality." In addition, her chest will hurt when Cordelia reacts to something.
At the start of the anime he gets irritated by Yui. Later, when Laito and the other brothers decide to play a dart game and Yui as the prize. Shu wins the challenge and tells Yui to stay away from him. Yui decides to apologise to Shu for being irritating and sees him in the bathtub. He pulls her in and sucks her blood. He is surprised at the finest quality blood in her. He also remarks that Yui is now hot as a vampire's fangs are at her neck and calls her lewd.
Yui first encountered Ayato sleeping on a couch located near the main entrance hall of the Sakamaki mansion. Ayato first drank Yui's blood at school the day after he met her whilst skipping class. He states that Yui's blood tastes "sweet", and it's the best blood he's ever tasted throughout his life. Ayato also tells Yui, she is his "special existence" in the world.
Because of Ayato's playful and rebellious attitude, the two will often get into little, unnecessary arguments which Ayato always wins in the end (although he tries to hide his embarrassment whenever Yui stands up to him). He also likes to pull pranks on her to make her jealous or to get her to confess her 'feelings' for him. He will always make fun of Yui, calling her names like Chichinashi (that literally means flat-chested) and scaring her, as well as forcing her to do things she doesn't want to. At one point, he wanted to be served 'hot blood', thus he began an attempt of dribbling melted candle wax all over her body so her blood temperature would rise. Fortunately, this attempt failed.
Ayato never understood why Yui called out for her father when she got too scared of 'moving dolls', as he laughs and jokes around why someone would call out for their parent. This shows that Ayato never received any of the unconditional, parental love from not only Cordelia, but his father as well. Ayato never knew what unconditional love was, as he never really understood how to love another person (Yui, being in this case) without it benefiting you. Ayato never understood the full concept of love until Yui appeared and began to live with them in the Sakamaki household.
At first he only thought of Yui as a food, and someone whom he can drink "first-rate" blood from. Ayato did not see Yui as an equal, in fact he did not see her as much as a person. He saw Yui as something he can play and do whatever he wants to with whenever he felt like it. When their relationship became more intimate, both Ayato and Yui began to develop feelings for each other, although he became confused by his feelings causing him to be frustrated and sometimes yell at her more.
Ayato becomes much more possessive and protective, as he doesn't want her interacting in any way with anything and anyone: both the Sakamaki siblings as well as friends and classmates in (or out of) school. Ayato is also against the fact that Yui is a believer of God. Ayato instead wants Yui to look up to him, and he is so jealous that he even envies things such as the stars, dogs, roses, and even food. He only wants her to have her eyes on him and only him. She starts to also fall in love with him and wants him only to suck her blood not anyone else's 'cause she confesses that she likes the pain given by Ayato. Ayato is also mentioned by Yui to be very handsome.
Like Ayato, sees Yui as a toy to play with. Since Laito was wrongly taught by Cordelia what love was, he has a perverted attitude. He enjoys making Yui do things she doesn't want to and to making her enjoy what she hates. He likes to irritate her as much as Ayato and Kanato.
Kanato has been seen to show yandere tendencies towards Yui, and is rather possessive of her.
Subaru has shown to be sensitive over Yui only because Yui kept him thirsty so he became too violent. Somehow. Subaru has a feeling such as to hate to Yui because she reminds him of his mental unstable mother Christa. Yui reminds him of his mother when she is acting worried and sweet. Other than that, because his mother often made him feel like he was filthy and unwanted . In the anime, it is suggested that he has fallen in love with Yui, as shown when Kou begins to taunt Subaru about Yui’s current predicament, and his when Ayato brings Yui back and he punches a wall in jealousy.
Kou is not completely a vampire when he was little it been tough he is pop star he does not understand the meaning of love. He was found in a well . When Yui came along , he , like Subaru and the rest of the Mukami brothers , felt comforted and someone caring for him and lost his virginity with her afterwards.
- Yui's clothing has been changing in ever part of the games (excluding Haunting Bridal and More Blood).
- Lost Eden is the only game with good endings that Yui doesn't get married.
- It's unknown who are her biological parents are or where she actually came from.
- Yui's name means "Italian aid, reliant help, solely helpful, helpful fountain, and helpful life.
- Yui's may not be her actual name as it possibly given to her by her adoptive father Seiji Komori.
- It's unknown if she's truly human due to the fact that she has demon heart in her chest and is capable of having it in her.
- She's 17 years old.
- Her blood type is O.
- It's unknown what happened to her biological parents or if they are still alive.
- Yui's the first character to be adopted, the second are her love interests out of the Mukami brothers.
- Yui didn't know that her adoptive father was a vampire hunter until she stayed at the Sakamaki household.
- She appears to have been sheltered throughout her life due to the fact that she has little to no knowledge of pop culture.
- Roses often symbolize Yui due to her love for the demon boys and her innocence as a human being.
- Yui doesn't seem to be interested in discovering her real parents.
- She's the first character to have an unknown birthday date and the second is Kino.
- She later became a masochist although she denies it.
- At the end of the first season, it may have been possible that Yui became a vampire because she commented that she is thirsty.
- In one of the drama cds, Yui tests higher than Reiji.
- Despite being a physically weak human, Yui managed to kill or capture the boys in some of the endings despite not having any kind of training of hunting demons.
- She's not a good swimmer which is shown in the second episode of the anime.
- Yui is said to have the ability to see ghosts since as a child.