He is Asago's father and the kumicho of the Silver Dragon Clan Sumongumi, the largest group under Kuryugumi. He is strict with her and always pressures her to excel in everything, emphasizing that her role as a seisai takes precedence over all else.
After Rui severs Asago's right arm, Sumon forbids her from seeking psychological assistance to cope with the adjustment to her new prosthetic arm, insisting that showing any sign of weakness is unacceptable. When Ryuuji gets shot and falls into a coma while protecting her, Sumon also prohibits Asago from returning to the Sumon household. However, some time after the engagement is called off, Asago observes that her father allows her to come back without any sarcastic remarks. She assumes that Ryuuji must have spoken to him.
