Kōta Mizuhara is a second-year student at Tokise High School and a member of the Tokise High School Koto Club. He is distinguished by his light blonde hair and big round eyes, initially standing at 170 centimeters tall, with some suggestion that he continues to grow over the course of the series. He often disregards school uniform rules.

Alongside his friends Saneyasu Adachi and Michitaka Sakai, Kōta is part of a trio frequently referred to as the three idiots. His association with them and with the main protagonist Chika Kudo began in middle school, when Chika helped him and the others after they were involved in a fight. Feeling a sense of gratitude and loyalty, Kōta followed Chika into the koto club primarily to support him, rather than out of an initial personal interest in the instrument itself. He is described as having a childlike and pure personality, often clinging to people he likes in a friendly, innocent way regardless of gender. He cares deeply about not letting his teammates down.

Kōta’s role in the club is that of a dedicated member who struggles with rhythm and technique, which causes him to worry that he is holding the group back. Despite these difficulties, he shows unwavering dedication and genuine passion for the club, steadily improving through persistent practice. His resilience and commitment to the team reflect his strong inner character, and his growth as a musician is a key part of his development as he contributes to the club’s journey toward national competition.