Plot Background

Zero was born to a prestigious vampire hunter's family. Zero, along with his identical twin brother, Ichiru, and Kaito Takamiya, were trained as vampire hunters by Toga Yagari who frequently looked after the twins when Zero's parents were out hunting. Yagari lost an eye protecting Zero, when the still naive Zero makes himself vulnerable to ex-human vampire when the nurse turned wild and tried attacking Zero due to her Level E state. This was Zero's first lesson on the danger of seemingly innocent vampires, Zero promises Yagari that he will never regret losing his eye to protect Zero.
Four years ago, Zero and Ichiru were taking a walk and Ichiru found Shizuka Hio sitting in a tree. Ichiru was amazed at how pretty she was, but Zero recognised as a vampire and told Ichiru to stay away from her. A few days later Shizuka attacked his family, she bit Zero and she killed his parents, Zero passed out believing she had killed his brother as well.

After that attack Zero was taken in by Kaien Cross. His temperament turned cold and hostile, and at times cruel to the young Yuki who reached out to him. Yuki approached him carefully and her gentleness began to win him over. Zero kept his affection for her to himself due to her loving Kaname Kuran. Zero failed to reveal that he was going to turn into a vampire after Yuki revealed she was afraid of all other vampires, but over the next four years, the two became friends and constant companions, Zero acting as Yuki's protector.