The adoptive daughter of the headmaster of Cross Academy, Yuuki is a member of the school's disciplinary committee (public morals committee in Japanese), which is the 'front' for the Guardians. Ten years prior to the events of "Vampire Knight," Yuuki was attacked by a vampire, but was rescued by Kaname. She has lost all memory of her life before the attack, and Headmaster Cross took her in as his own daughter. Yuuki loves Kaname, and even though she has witnessed his vampiric nature when he bit Ruka, she still couldn't stop loving him. However, she has also started to let Zero, a childhood friend, drink her blood, hoping to save him from madness. Yuuki is cheerful and easygoing, and thanks to her work as a Guardian she is good at keeping her cool in sticky situations. There have been hints that she is special, given that vampires are especially drawn to her blood. Her weapon of choice is the "Artemis Rod," an extensible staff she received from Headmaster Cross.
Eventually, it is revealed that Yuuki was born a pureblood vampire, a member of the Kuran Clan. Her mother had sacrificed herself to seal Yuuki's vampire side, as well as her memories. Yuuki's existence was hidden from the other vampires. She was kept in a basement in the Kuran Mansion without any windows in order to protect her from both her parent's older brother, Kuran Rido. It was when he attacked the Kuran family, seeking to take Yuuki, when her memories and her Vampire side were sealed.
This also makes her Kaname's sister, and her intended fiancee, meaning Yuuki was born to be his wife.