Hiei

Hiei was born from quirk breeding, straight into a cult/group that lead an underground fighting ring for powerful quirks. They breed to get better, more flashy quirks to fight in their ring. They tend to breed within their group, but if they must, they will also take others into their group (by force) in order to breed stronger quirks. Hiei was the result of warp quirk father, and a transformation/mutation quirk mother. Before his parents were forced together, they didn't know each other. His father joined the cult with his warp quirk, and his mother was taken into the cult for her type of transformation/mutation quirk. Before Hiei there were two still born and a child that died in infancy. They didn't have much hope when Hiei was born, but he was the quirk combination they had been looking for. 

Hiei holds a lot of traits from his mother. obviously his transformation/mutation quirk, but also his soft face, dark eyes, and black hair. His skin ton was received from his father, grey and black. 

His personality is kinda all over the place. Naturally, he is shy and patient, and would rather curl up in a dark space with a warm blanket than start any kinda fight. 

Of course, when he's being forced into the ring, his "handlers" have a way to jolt the shy out of him. They force Hiei into a cage that's barely big enough for him. They take a "shock stick" or a long pole that has a taser on the end. They use this shock stick to get Hiei angry and fiery. Then they attach the opening of the cage to to the caged ring, and when they open the cage, the only place for him to go is the ring. Now that he's riled up and angry, he's nearly blinded with blood lust. 

After the match the only way to calm him down is to either drug him, or for someone to use their quirk to subdue him. This is where Tomeru comes in.

Tomeru has a quirk that helps end matches. He makes it possible to drag Hiei out of the ring without a fuss and put back into his cage. Hiei at first doesn't trust Tomeru because of this, but he quickly realizes that Tomeru isn't there to hurt him, in fact he's a captive himself.