Zero is the second prefect of Cross Academy. It appears as a young man with character cantankerous, hard and unsociable. It is the kind of person who suffers himself and still lonely. It fulfills its directives as prefect of discipline and its role as guardian of the secret of Cross Academy, "which have been entrusted by the director reluctantly, but so zealous. That is why he is feared by all the young girls of the Day Class , except for Yuki.
It is half-vampire, half-hunter. When he was a young boy, he was bitten by Shizuka Hio, a pure blood vampire. Zero also had a twin brother, Ichiru. Their parents were vampire hunters. They were killed by Shizuka's revenge because they had killed the man she loved. Having fallen under the spell of the vampire Shizuka, Ichiru then fled with her and Zero was believed dead, which fueled his desire for revenge against Shizuka.
Despite the circumstances of growing closer to Yuki that he disagrees with it the ideal peaceful between humans and vampires that has been taught by their adoptive father. Since the massacre of his family, Zero also devotes an intense hatred for vampires who he sees as "monsters in human shape" - why he would have hated himself for years, until impulses take over: he bites when Yuki.
When Yuki becomes a vampire, he promises to meet again several years later and kill her as a vampire hunter. In the manga, he kisses her before she goes away with Kaname. In the manga, one year later, he still seems have feelings for Yuki, because he seems to feel some jealousy towards Kaname when he sees the arms of the latter.