Rumika Aina, the 30-year-old homeroom teacher of Class 2-B at Koyo Women’s Academy, mentors students like Hibiki Sakura and Ayaka Uehara while navigating personal anxieties about marriage. Her blunt remarks on age often irritate peers, contrasting a reformed delinquent past marked by underage drinking. Physically, she stands 1.64 meters tall with a toned 55-kilogram frame, long blonde hair, yellowish-brown eyes, and a cultivated tan, her 80-58-82 measurements reflecting a practical focus on fitness.

After discovering colleague Satomi Tachibana’s lost Silverman Gym membership card, Rumika joins the gym alongside teacher Yakusha Kure, dedicating herself to abdominal workouts like side bends and bicycle crunches under trainer Machio Naruzo’s guidance. Unfazed by Machio’s extreme musculature, she bonds with Satomi as the "Chubby Butt Duo," blending camaraderie with fitness goals.

Rumika’s friendship with Satomi thrives on shared interests in cosplay and marriage pursuits, while she quietly admires Yakusha’s athletic confidence, juxtaposed against her own body-image doubts. A school hiking trip exposes her inexperience, leading her to panic after getting lost with Hibiki and Satomi, revealing vulnerabilities beneath her composed exterior.

Though gym activities gradually ease her self-consciousness, her growth remains subtle, framed by comedic mishaps and situational humor rather than deep narrative exploration. Her story threads through lighthearted interactions, maintaining a secondary role within broader ensemble dynamics.