First zero is not a cold boy always exist weather he lived besides his broather alone but shizuka later of hatred killed his family except his broather and it has bitten causing injury physical and mental making it in a vampire level E.
Found kaien he living with yuki in クロス academy, bite made him hate the ヴァンパイア and because he hate vampire he hate and night class and this is why he was decidet be a vampire hunter.
Trained によって yagari and teaming with yuki he guarded academy によって means of bloody rose his weapon.
But as epected vampire behavoir he had made to bite his lover not allowed Yuki.