Zero Kiryu's twin brother Ichiru was, too, trained by Toga Yagari to be a Vampire Hunter like his parents. However, unlike Zero and his prodigal abilities, Ichiru was sickly and weak, which made Zero worry over him endlessly. One night, he overheard his parents saying that they did not know what to do with Ichiru because he was not suited to be a fighter, not like Zero. Though Zero loved his brother very much, Ichiru began to resent Zero and his parents. After hearing what his parents said, Ichiru ran outside and met a crying Shizuka Hio, who sat in a tree mourning her dead lover. In his pain, his confusion, and his anger, Ichiru connected with Shizuka.
After the death of his parents, Ichiru became a loyal servant to Shizuka and hid his survival from his brother. His hatred for Zero only festered, so much so that he tossed to Shizuka a sword with which to kill Zero. He still resents Zero for the fact that his parents did not hold him in the same regard as they did Zero.
According to the words of Kaname Kuran, twins from a Hunter family are extremely rare, even to a vampire that has lived for centuries. This is due to the fact that when Hunter fetuses are in the womb, they rely on instinct as they would in battle. This means that fetuses fight for supremacy and eat the weaker of the two. The fact that Zero and Ichiru were able to be born together is highly unlikely. There is evidence to suggest that the reason why Zero is more adept at vampire hunting and more healthy as a whole than Ichiru is because they were meant to be one, and Zero is simply the greater half. By one twin consuming the other, they become one, as they should have been in the first place.
Ichiru arrives at Cross Academy with Maria Kurenai, but does not reveal himself to Zero until 18th Night. His revelation shocks Zero, but Ichiru believes that he knew all along. He expresses jealousy of Zero, because Shizuka refused to turn him into a vampire and shows too much interest in Zero.
After Shizuka is killed, Ichiru vanishes for a time, only to reappear as a student at the Academy in 30th Night. He is, of course, immediately popular in the school, a bright contrast to his gloomy twin. Ichiru makes disparaging remarks to Yuki about his brother's poor taste in women, to her embarrassment and later warns Zero not to fall in love with her.
Later, it is shown that, despite appearances, he is working for the council and aiding Rido Kuran, he is in fact seeking the healing of Rido, so that he can be fully killed. When his attempt goes awry, he seeks Zero once more and asks him to consume him, so that he can once more become a part of his brother. Ichiru's dying wish, though, is for Zero to live on and complete his goal.