Kiryuu, Zero

錐生零

Zero is Yuuki's childhood friend and a student at Cross Academy. He serves as a Guardian and is also a member of the disciplinary committee. Zero lives with Yuuki and Headmaster Cross, who acts as his legal guardian. His family, a line of vampire hunters, was murdered by a pureblood vampire when he was young. He harbors a deep hatred for vampires and claims to have taken on the role of Guardian to learn to hunt them better.

Spoiler

In truth, Zero is a vampire that was once human. He was bitten by the pureblood vampire, Shizuka Hio, during the attack on his family. He kept the secret that he was a vampire from Yuuki until he was no longer capable of controlling his vampiric urges.
Zero's body rejects the blood tablets that most vampires take to control their hunger. Drinking Yuuki's blood helps him maintain his sanity, but he is destined to become a Level E vampire eventually. He has started to accept his feelings towards Yuuki and has gone as far as to almost kiss her. Zero's weapon is the gun "Bloody Rose," given to him by Headmaster Cross. He made Yuuki promise that if he becomes a Level E vampire, she will kill him with that gun.
Zero's twin brother, Ichiru, arrives at Cross Academy, with Shizuka Hio, in order to seek more power against Rido Kuran. It is later shown that Ichiru was involved in the planned attack against the Kiryuu Family. Despite learning this and his conflicting feelings toward Ichiru, he still loves his little brother.
Later on in the series, it is revealed that Yuuki is actually a Pureblood vampire from the Kuran. After this revelation, Zero felt betrayed as he felt that Yuuki had been toying with his emotions. During Rido's attack on Cross Academy, Ichiru requests that Zero drinks the remainder of his life, as he is on the verge of death. In doing this, Zero acquire the full amount of power that he should have had from birth, in exchange for Ichiru's life. Before Yuuki's leaves Cross Academy, Zero states that one day, he will come to kill her, as he still wishes to hunt all vampires (in the manga he also kisses her farewell).
After the one-year time skip, Zero has been living off of Blood Tablets and continuing to be a hunter. He reluctantly goes to classes at Cross Academy, but cannot stay in the dorms due to his uncontrollable hunger. At night time, Zero would hunt down vampires that were on the wanted list and would drink their blood to quench his thirst. This helps him control himself without Yuuki's blood, but he still finds his thirst for blood difficult to manage. When reuniting with Yuuki, he is distant and cold to her in the beginning. However, it is shown that he still cares about Yuuki and still assists her when needed.
After Kaname threw his heart into the furnace, he tells Yuuki and Zero that they should be together. After the war, Zero hugs Yuuki and tells her to let go of everything that has happened. He also decided not to run away from his own feelings anymore, and that he'll wait for Yuuki to say that she'll go with him even if it'll take years for her to say it. After a thousand years, it is seen in the manga that Zero and Yuuki had a happy family life and raised two children together. Yuuki stayed by Zero's side until his last breath, and after, she went to his family's grave and left Zero's bloody rose (as his own grave because he turned to dust) while reminiscing her memories of Zero. She said that though they had a lot of quarrels, Zero really made her happy.