Sumio Matsunuma is a 19-year-old illustrator residing in Karuizawa alongside his childhood friend Kouhei Aizawa and his older sister Kaoru Matsunuma, confirmed as his sibling. His romantic pursuits face consistent challenges, frequently overshadowed by Kouhei's interactions with female characters. This recurring dynamic fuels his personal frustrations within the narrative. He develops a romantic interest in Eri Tsunoda, another character in the story. His role supports the central plot involving Kouhei, Kaoru, and Noriko Kuonji.