Chief of the Special Mobilization Unit at the AMO Kanto Branch. He knew Akira Gunji's father. He recruited Akira into AMO for reasons related to his father and ordered Sho Tenkubashi and Akira Gunji to form a party for missions. As a high-ranking official, he often gives 'orders.' He claims to be one of the survivors of the Hakugin attack nine years ago. He is the one who brought Sho Tenkubashi—who also survived and gained 'super-regeneration' abilities—into AMO, trained him, and led him to live as a 'weapon.' He says he lost his right hand and eye in the fight against Hakugin, the one who killed Akira's father; he was present when it happened but was unconscious and unable to help. His right hand is a prosthetic, and he has a large scar reaching up to his collarbone, which is why he always wears turtlenecks. Based on the author's comment that the necklace Reiji wears is important, it is speculated to be related to the tragedy of Hakugin. He uses a longsword as his weapon and is called the strongest in the Special Mobilization Unit. Among certain illegal aliens, there is even a rule that says 'if you encounter Reiji Amamiya, retreat immediately.' He is depicted as being exceptionally handsome. In episode 16, when Reiji waited for Akira at the school gate, female students whispered about how handsome he was, how intense his aura was, and wondered if he was a model. He likes dogs and used to have one, but it has since passed away. While he appears kind, he is an enigmatic character who occasionally shows a very cold, chilling side and acts in mysterious ways.

A member of the Special Mobilization Unit at the AMO Kanto Branch. He serves as Reiji Amamiya's aide. In official settings, he calls Reiji 'Amamiya-san,' but when they are alone, he uses his first name, suggesting a shared past. His weapon of choice is a large-caliber sniper rifle. He possesses traits such as being peerless, wearing suits and glasses, and having sharp, sanpaku eyes.

After the incident nine years ago, Reiji Amamiya took him in and brought him to AMO, where he was trained by Reiji to not fear pain. Not only the memories of that time, but also Reiji's words—'You will become the ultimate weapon'—have remained an obsession for him. He uses dual swords. He chose this weapon because he thought two would be stronger than one.