Natsumi Koshigaya, youngest daughter of Yukiko and younger sister to Suguru and Komari, lives in Asahigaoka's rural village. She attends the local branch school as a first-year middle school student. Standing 155 cm tall, she sports distinctive orange-red hair in a messy half-up style and blue-grey eyes, favoring practical, boyish clothing for ease of movement. Her fun-loving, carefree personality drives frequent pranks, especially teasing Komari about her height. Rebellious and academically struggling, Natsumi talks back to her mother and deflects blame for misbehavior onto others or circumstances. She cringes at a childhood memory where her brother complex led her to declare she’d marry Suguru. Natsumi shares a close bond with Hikage Miyauchi, Renge’s Tokyo-based sister who visits Asahigaoka periodically. They spend significant time together during these visits, their friendship rooted in Natsumi’s childhood view of Hikage as an older sister figure. She also befriended Aoi Nizato during a school trip to Okinawa, bonding over their shared age and mutual interest in badminton. Within her family, Natsumi’s dynamic with Komari blends teasing with underlying loyalty, while clashes with her mother stem from her rebellious streak. Her hobbies include badminton—her connection point with Aoi—alongside outdoor adventures and countryside explorations with friends.