Seishiro Sumikawa, nicknamed "Tori," is a classmate of Oriko Seki, known for his short brown hair and brown eyes. The nickname stems from his pet bird ownership. He possesses a naturally sweet disposition and finds himself drawn to lively girls. His amiable personality, combined with strong athletic skills, makes him popular among classmates. During a peer discussion about favorite idols, he expressed a preference for energetic types like Oriko.

While cleaning the torii gate one evening at the shrine where he lives, Seishiro encountered Oriko. He mentioned previously observing her at the shrine with another boy, later identified as Uriken. After Oriko explained Uriken's background and described him as occasionally annoying, Seishiro complimented her maturity in discussing such matters openly.

Seishiro joined rehearsals for the town's annual Shinto spring festival ritual dance. When Matsuki Akino criticized Oriko's innkeeping capabilities and pressured her to withdraw from the performance, Seishiro intervened. He defended Oriko's right to participate, pointing out both girls were selected specifically as future inheritors of their family inns. As Oriko and Matsuki's argument escalated into a heated exchange of insults, he repeatedly attempted to mediate, displaying visible embarrassment and dismay through gestures like facepalming and verbal interjections such as "Hey, come on..." and "Oh, good grief."