Ichiru is a very particular character, I'd define him the best among all of this series, becaue he had an iter that he wanted to make and he did it. He is presented as the twin brother of Kiryu Zero, in the moment in which Hio Shizuka revived.
Shizuka was the person who gave him trust and that convinced him to live. When Ichiru was a little child, he loved Zero very much, he was always very close to him, but when he heard his mother talking about his future as hunter, he suddenly changed. Unfortunately, Ichiru was sick and his health was very bad, so he won't ever become a hunter, even if he wanted to. Once that he knew Shizuka, he saw that they had the same expression, realizing that the two of them were very similar, so he helped her to get her revenge, letting her killing his own parents and biting Zero, the brother that he loved so much.
As I previously wrote, after many years, he came back with Shizuka, in order to get his revenge toward Zero. Thanks to Shizuka's blood, he was able to gain a better health, and he was almost healed, so that he could fight at his best against his brother. Unfortunately, he couldn't do it anyway, because when he knew about Shizuka's death, he left... but he came back after a bit, even if this time, he did it because he wanted to get revenge from another person: Rido Clan.
He attended the Cross Gakuen and he was very well-loved from girls (argh!!), but his goal was to divide Zero and Yuki, in a first time. Things went suddenly worse for him, because he tried to fight against Rido even if he knew that he surely wasn't strong enough for him, and, in fact, he was defeated by him. Since Zero was about to become a level E, he made the possible for him, and told him to drink his blood as long as he could do it. He will die in his arms, but happily, because, finally, he could reach Shizuka again and could be with his brother until the end.
His personality is in one word... vengeful! But this is not the real Ichiru. Inside of him, he has always been a very kind person, one that would give the all the things he owns to the ones he loves. Unfortunately, due to what he heard that night, his illness and the continued comparison between him and Zero he will be lead to become what we saw in the manga (but he won't ever be bad or evil).
He's a very serious person who doesn't usually like to joke around, he just wants to grant the wish he always had, and he improved himself at his best to do it. The adjective that I think describes him better is determination, because, thanks to it, he became the wonderful person that we knew. Many times he's kind and shy too, especially with Shizuka and Maria.