Hirose Yuichi is the best friend of Taisuke before the first suicide wave which was when got Hirose his abilities. He left his home town to join the comrades.To find Akuro's heart,(A powerful orb which gave the comrades their powers) it must be taken in by a certain comrade, a strong comrade who could withstand its immense power. Hirose was the container chosen to hold the heart.
Hirose was a lonely child who lost his father and lived with his mother. Because it was only them two of them hirose wanted to do anything to protect his mother. Hirose met Taisuke in pre school. Hirose heard about how Taisuke lost his parents and felt sorry for him. When Taisuke got detention for acting out Hirose stayed back to talk to Taisuke. Hirose said he understand's how Taisuke was feeling. That was the day they became best friends.
Hirose gains the power of void; this allows him to shoot a ball that makes anything disappear. After gaining his power, with Katsumata's mind alterations, Hirose's personality changed into that of a destructive and ruthless sociopath, and he murdered many people. After absorbing the Heart, Hirose revealed that he desired to kill every other life form on Earth, making a world for himself. When he absorbed the Heart, his void increased greatly, to the point of hollowing out an entire mountain in one blast. His body becomes intangible due to being the void itself.
Hirose would never hurt a living thing, he wants to treat people the same way he wants to be treated. But when he gains his abilities he becomes a different person. He's usually very calm and collected and almost never gets angry. He also has no problem with killing people.