A cute, chipper brunette hiding a very sad past, some of which she herself isn't really aware of.
When the series opens, Yuuki has no memories of her past beyond when she was rescued from a vampire attack by another vampire named, Kaname Kuran. She is adopted by Kaien Cross, a former vampire hunter who allies himself with Kaname to form the Cross Academy in an attempt to prove that vampires and humans can coexist. She grows up as Cross' protoge and a student of the academy, where she also is a prefect. As such, she works to ensure the peace between the Day and Night classes by making sure the Night Class' vampires don't feed on the Day Class.
Some time after being adopted by Cross, he adopted another victim of vampires: Zero Kiryuu, from a family of vampire hunters. Yuuki cares for Zero and attempts to bring him out of his shell, and to convince him of Cross' beliefs that not all vampires are evil. Zero, however, refuses that it's possible and also hides a secret: he is slowly becoming a vampire as a result of the attack he and his family suffered so many years before. Yuuki is at first distressed but eventually decides to help Zero by offering him her blood, which he drinks on several occasions. Zero makes her promise that if he ever loses his mind and becomes a vampiric monster, that she will kill him herself.
In chapter 35 of the manga, Yuuki's past is revealed: she is, in fact, the younger sister of Kaname Kuran and daughter of the deceased Haruka and Juuri Kuran. She and Kaname are two of the only remaining Kuran pureblood vampires. Their mother gave her life to seal Yuuki's vampiric powers and memories, and Kaname has acted as her protector against their uncle, Rido Kuran, ever since. Kaname hopes to continue their relationship as an engaged couple, but says that it will only be that way if Yuuki so chooses. Zero reacts badly to Yuuki's new identity at first, although since she's a pureblood his drinking of her blood may prevent him from descending into insanity as his own vampiric tendencies take over.
Yuuki Cross is created by Matsuri Hino, and the inspiration for her character results from the Kanji of Yuuki Cross and author's desires. Her early character concepts, Hino comments that she wants Yuuki to be different from her other main female protagonist. She designs Yuuki to have a passion in justice while having a tender, nurturing personality.
Kanji and Romaji meanings:
On the exterior, Yuuki is cheerful and spunky, in spite of often not getting sleep thanks to her duties as a prefect. She is ironically both maternal (as shown through her attitude towards the emotionally injured Zero), and naive (almost to the point of babyish - as shown by her interactions with Kaname), but has a heart of gold. However, during most of the manga's action, Yuuki struggles to find a peaceful and pleasant solution for everyone and gets frustrated when it seems to be impossible. She is an idealist and hates it that Zero and Kaname seem incapable of getting along, and her being kept from secrets does not help her. After her transformation into a vampire, and perhaps due to her disadvantageous circumstances of being thrown into the center of royal vampire politics, she becomes less assertive (not that she was previously aggressive by any means) and seemingly only manages to flow with the events that are brought upon her.