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An emissary of Lord Tamamo who had his left eye replaced with a sesshouseki and searches for a worthy successor to hold the Kyuubi. Able to use spiritual insects and uses the sesshouseki to manipulate people.
Apparently, Mitogawa was the son of a slain exorcist mother. In his grief, he wanted to find a way to bring her back using the Kyuubi (ed note: still looking for info on this in the series).
During a trip to the Vatican, he managed to find an entity calling itself Lord Tamamo, and she replaced his left eye with a Death Stone (or sesshouseki) in return for his service to her.
In Ga-Rei Zero, he appears to be manipulating events behind the scenes for some unspecified purpose other than trying to find someone worthy of holding a Death Stone within themselves. He killed the freelance exorcist Mei and revived her using a Death Stone, but he deemed her unworthy to hold it after she was killed by Yomi Isayama. Mitogawa decided to switch his focus to Yomi instead, first severing all her nerves in order to make her feel helpless as her friends take pity on her or vent their frustrations at her. Then, he snuck into her hospital room and completely healed her with the power of the same Death Stone, using her own grief against her as he turned her into an unwitting pawn in his scheme.
In the Ga-Rei manga, Mitogawa occasionally pops up as an emissary of Lord Tamamo. He doesn't directly cause trouble for the exorcists, and seems to be little more than an observer. He appears to be working for the Juugondou at one point, but reveals that he is using both them and the exorcists to find a worthy successor of the Kyuubi for his master.
Mitogawa appears to have control over spirit insects, and cannot be physically killed by normal means. Both Mei and Kagura's father have tried to attack him, but he simply turns into a cloud of blue-colored insects and reappears somewhere else.