Yuuki's childhood friend, Zero is also a Guardian and likewise on the disciplinary committee. Yuuki has been taking care of him as her little brother since he was brought into their home (the Headmaster's). His family, a line of vampire hunters, was murdered by the pure blood vampire Shizuka Hio 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.