Masamune Shichigahama, a third-year at Sōshūkan High School (Ao High), captains the boys' rhythmic gymnastics team. His passion for the sport ignited when he witnessed coach Shuusaku Shida perform a park backflip, leading him to recruit childhood friends Keisuke Tsukidate and Nagayoshi Onagawa to found the club.

His family runs a long-established sasa kamaboko shop, a Miyagi fish cake specialty. Close bonds, especially with Tsukidate and Onagawa dating back to elementary school, define his relationships. As captain, he exhibits passionate leadership and unwavering dedication, though he struggles to convey technical concepts clearly, sometimes relying on vague descriptions or sound effects.

Specializing in expressive freehand routines requiring no apparatus, his leadership proactively encourages unity among members. He reacts strongly to any perceived disrespect toward the team or school. A recurring rivalry exists with Toru Takase, captain of rival Shiro High School, frequently escalating into heated but non-malicious confrontations.

His development focuses on maintaining team morale during competitions and overcoming challenges like recruiting enough members. His leadership is tested in tournaments against strong opponents, including Shiro High. The anime film continuation depicts his captaincy during the Inter-High competition, where his focus remains guiding the team toward collective achievement, not individual recognition.

His name "Masamune" combines characters meaning "government" and "religion/sect/essence," interpreted as embodying the team's strength, while "Shichigahama" translates to "seven beaches," reflecting his coastal Miyagi origins.