Zero Kiryuu (錐生零) The silver-haired scion of a family of vampire hunters.
Born to a family of vampire hunters with his twin Ichiru, Zero was the healthier, happier twin. Ichiru, by contrast, was weak and eventually detached himself from the family. Ichiru became friends with a vampire, who bit Zero and Ichiru and slaughtered the rest of the family. Ichiru happily joined the vampire, Shizuka, while Zero fought his transformation and declared himself to be an enemy of all vampires.
Zero was taken to and adopted by Kaien Cross, who cared for him along with Yuuki. Zero eventually fell in love with Yuuki and is constantly torn by his desire to drink her blood and his desire to protect her. When she learns that he's slowly becoming a vampire, Yuuki allows him to drink her blood, and he feeds from her semi-regularly for some time.
Ichiru eventually comes to the school and confronts Zero, and Shizuka comes as well. Kaname kills Shizuka and drinks her blood to take her powers, a death which is eventually blamed on Zero. In the manga's current state, Kaname--with whom Zero often conflicts--has high hopes for Zero, who has now drunk the blood of three purebloods: Yuuki, Kaname, and Shizuka (via Kaname). As such, Zero is a vampire hunter and a vampire himself, now with a phenomenal bloodline. Kaname hopes that Zero will defeat Riido to protect Yuuki, who Zero still loves in spite of her newly-vampiric state. However, the Hunters Association has locked Zero up.
Matsuri Hino, who created Zero Kiryuu, designed Zero to have a casual look in contrast to Kaname's formal looks. She and her editor are concerned about Kaname's character overshadowing Zero, so they changed his name to get a stronger meaning and give Zero, a rebellious nature. Zero's Japanese voice actor is Mamoru Miyano and his American voice actor is Vic Mignogna. He first appears in Vampire Knight volume 1 and episode 1.
Externally, Zero is a stoic character, never reacting to the antics of his classmates. In most of the action of the manga, he is bitter, angry, and impetuous. He is fiercely protective of Yuuki in spite of being one of the threats against her, often having to drink her blood because of uncontrolled thirst. Below his seemingly unfriendly and silent nature, he is deeply tormented by his vampiric/hunter identity and harbors a deep hatred towards himself and vampires, leaving him quite conflicted, especially after Yuuki turned out to be a pureblood. Despite this, his true nature and innate kindness shines through in the most unexpected moments, even if he views himself as a despicable creature.