Absolute Anime

Mitarai was a boy who was picked on endlessly and cruelly at school. He was a loner and didn't have much self esteem. He found one person who accepted him, Sensui. He showed Mitarai the worst of humanity and Mitarai joined his Jihad. Mitarai controls water; with just a drop of blood he can create a monster of water that cannot be destroyed (it just rebuilds itself). The more blood, the bigger the monster. It is in it's own special kekkai, a created alternate universe that cannot be broken by normal means.

Mitarai was given the ability to create monsters from mixing blood and water. The more blood, the bigger and more viscious the creature. Being an outcast, having no friends, and always being bullied made him give in to Sensui's offer.

When he attacked Kuwabara, he tried to use his monsters to keep Kuwabara's friends as hostages. But eventually Kuwabara beat him and didn't kill him. Although he did bring him back to meet the others, which after an attack from Sensui and Kuwabara being kidnapped, he finally revealed Sensui's lair to the group.