Kōta Mizuhara attends Tokise High School and is a dedicated member of its koto club. Standing 170cm tall at the series' start (with potential for growth), he sports light blonde hair and round eyes. His style is casual and non-conformist, typically featuring checkered brown uniform pants paired with colored hoodies over his shirt and non-regulation shoes instead of standard brown school footwear; he also carries a personal backpack instead of the school bag. Mizuhara only dons the full school uniform for koto club performances.

His personality radiates cheerful, childlike energy marked by sensitivity. He openly expresses affection, frequently hugging people he likes, a gesture stemming purely from fondness. Mizuhara comes from a family with three doting older sisters: the eldest is an office lady, the middle sister a dentist, and the youngest works front desk at a fitness club, fostering his somewhat spoiled demeanor.

Mizuhara favors spicy food, counting habanero peppers as a favorite, while disliking raw fish. A documented distinctive trait is his habit of wearing a bunny-themed hoodie.

Prior to joining the koto club, Mizuhara had a history of delinquency, shared with fellow club members Chika Kudo, Saneyasu Adachi, and Michitaka Sakai. He belongs to this group of former delinquents who become committed members, contributing to the club's "Ragtag Bunch of Misfits" dynamic.