Yuichi Hirose is Taisuke's best friend. He is small and timid for his age and is picked on by bullies. Taisuke and Megumi often help him out with bullies and animals, as seen in the omakes. Although small, he himself is infected with the virus and is given powers. He lives with last living family member, his mother. His mother soon commits suicide due to the virus and he ends up congratulating her on her death due to mind altering by a police inspector. Hirose gains the power to make anything "disappear" by creating holes(this ability is known as void). He later resents Taisuke and becomes ambitious and cruel.
After gaining his power, with Katsumata's mind alterations, Hirose personality changed to 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, but currently his goal is unclear. When he absorbed the Heart, his void increases greatly, and his body becomes intangible due to being the void itself.Source: Wikipedia