Yuki is the adopted daughter of the headmaster of Cross Academy, a 16 year old first year student who wields the Artemis staff, an anti-vampire weapon formerly used by her mother. She begins as a human, a Guardian protecting the human students from the vampire students. Yuki is close friends with Zero Kiryu, who she becomes determined to help once she discovers he had been turned into a vampire, following which their friendship rapidly evolves.
She has been in love with Kaname Kuran, a pureblood vampire, since he saved her as a young child, but her affection is later divided to include Zero as well. She is generally a kind, caring, cheerful, and comedic character, but often portrayed as not being very smart, being a poor student due to the fact that she is a guardian at night and student in the daytime, giving her little time to study and sleep. She is often kept ignorant by other characters who know more about goings-on, which results in her character feeling guilty for being unaware of the suffering of other characters. Hino Matsuri has described Yuki as a healing and soothing character, who exemplifies her justice side.
In the manga, Yuki admit to Kaname that she needs the blood of Zero that she still loves to alleviate the sufferings of his absence. In fact, all pure-blooded vampires need the blood of the person they love.