Zero Kiryu

Zero Kiryu is Yuki Cross childhood best friend. Zero is also a Cross Academy Guardian and likewise on the disciplinary committee.

Background
Yuuki has cared about him as a close friend ever since he was brought into their home (the Headmaster's). His family, one of vampire hunters, was murdered by the pureblood vampire Shizuka Hiou when he was young. He harbors a deep hatred for vampires and claims to have taken on the role of Guardian to learn how to hunt them better. In truth, Zero was bitten during the attack on his family and therefore was turned into a vampire, something that has been kept secret even from Yuki until his vampiric urges began to surface and became uncontrollable. As Zero's body rejects the blood tablets that most vampires take to control their hunger, Yuuki began letting him drink her own blood, which helps his thirst for blood, but he is ultimately to become a Level E vampire. The story implies that Zero harbours romantic feelings for Yuuki. In chapter 35, he discovers Yuuki is a vampire, and in chapter 37 he aims his gun at her, which leads to Yuuki telling him her vampire side "ate" her other side. He almost kissed Yuuki twice showing that he has feelings for her.

Zero is currently being locked up by the Hunters Association. In chapter 39, it is implied that Zero might be a significant person who is crucial to the defeat of Kaname and Yuki's nemesis. As Kaname said himself, Zero is a vampire hunter with an excellent lineage being at the same time a vampire himself. And thanks to having taken blood of three purebloods, Yuki, Kaname, and Shizuka he has the power to kill the "king" of pure bloods, Rido Kuran. Something that Kaname can do but unable to, for Kuran Rido is his master who resurrected him. Zero's new powers are shown to be manipulation of vines within his body and he has possibly also absorbed the Bloody Rose, as he produces it from within his body. Zero calls Yuuki his enemy, but the two guardians now confront Rido Kuran together.

Rather than agreeing with Yuuki's desire to protect him as well, Zero pushes her out of the way of Rido's attacks and reveals his intention of killing Rido. Rido then reveals to Yuuki that Zero is just as bad as him, and that he devoured Ichiru for the sake of power. He then pushes Yuuki off the roof, not caring for her safety as much as she was in his way. A battle takes place off panel in which Rido is forced to retreat, and appears behind Yuuki. He begins to call her Juri and asks her why she can't love him. Zero then finishes off Rido with his enhanced Bloody Rose, ending the powerful vampire's existence.

Zero then reveals his intentions to wipe out all pureblood vampires, and points Bloody Rose at Yuuki, with every intention to end her life.

Triva

 * Voiced by: Mamoru Miyano in the japaness anime