Seiya is Moriya Ririka's next-door neighbor and childhood friend. The son of a doctor, Seiya aspires to follow in his father's footsteps. He is about the same age as Ririka, and they are in the same 4th grade class. While Seiya is not good at studying, and is the class clown, he is actually very smart and is good at coming up with strategies for the battles he and Ririka get involved in. Seiya is quite mischievous and enjoys playing video games, sports and teasing Ririka but is very honest with himself about his feelings for Ririka and will go to any lengths to protect her. Early in the series Ririka and Kanou-senpai hide their secret war from Seiya, but he eventually discovers their secret. Eventually Kanou-senpai gives Seiya part of his powers and entrusts his mission to him, making Seiya Nurse Angel's main supporter.