He is one of the bullies that bullied Theo and was then expelled from school thus becomming an outcase of society, so he decided to become a serial bomber so that he can literally "blow up reality". He later shows up at Yokohamabane High School to blow it up after blowing up the arcade where the pictures were taken and 5 other locations. However, Nice and Murasaki spoiled his plans so he activated the bombs that were strapped onto him, but he was stopped once again by them.
A month before the current story he and four other High School guys bullied Theo because he claimed that he was only doing what the others did, when Yasuo Kitazawa exposed the five of them, they were expelled immediately.
Powers & Abilities
Being a Minimum Holder, Kojima possesses supernatural abilities of time acceleration. This allows him to manipulate time by making time on any device move faster. For example, a bomb that takes a while to explode after activation can be detonated in trice the time with his ability. He claims that even a bomb can be a grenade. In order to use his ability he most likely needs any device that has a timer on it such as a bomb, watch or even a microwave.