Nagayoshi Onagawa is a third-year student at Ao High School in Miyagi Prefecture, where his family operates a fish shop. He co-founded the school's boys' rhythmic gymnastics club with his childhood friends, Masamune Shichigahama and Keisuke Tsukidate. Initially joining without strong personal ambition, Onagawa gradually embraced the sport. His natural flexibility and proficiency in tumbling, recognized by teammates and the coach, led him to aspire to become the team's ace.

Onagawa possesses a cheerful and lively personality, serving as the team's primary mood-maker who often lightens the atmosphere with jokes and humor. He habitually assigns humorous nicknames to teammates, particularly first-year members Shotaro Futaba and Ryoya Misato. He also harbors a strong interest in idols and cute girls, notably admiring and supporting Masami Kudo (Ma-chan) from the idol group Tan Tan Girls.

His character development centers on grappling with his role following Misato's arrival. Previously comfortable with his status, Onagawa began experiencing self-doubt upon recognizing Misato's superior talent. He jokingly referred to Misato as the "Ace" while privately struggling with feelings of inadequacy and displacement. This internal conflict led him to feign an injury to avoid practice, reflecting his uncertainty about his place on the team. He later found renewed motivation after witnessing the perseverance of his favorite idol, Ma-chan, and receiving direct encouragement from Misato. This helped him refocus on contributing to the team's collective strength rather than pursuing individual recognition as the ace.

Onagawa's gymnastic abilities emphasize exceptional flexibility and tumbling skills. He continues to train and compete as part of Ao High's team, participating in events like the regional tournament aimed at qualifying for the Inter-High competition. His journey illustrates a shift from seeking individual validation to embracing teamwork and finding personal significance within the group dynamic.