Yōjirō Mutsu is a third-year student at Shiro High School and vice-captain of the boys' rhythmic gymnastics team. Born in Tokyo, he relocated to his mother's hometown of Aomori during childhood, forging a lasting friendship there with teammate Tōru Takase.

He possesses mid-length, straight mauve hair, styled with long bangs swept mostly to one side. His small brown eyes are frequently hidden behind closed eyelids. His standard outfit is a purple and white tracksuit zipped to the collar; for practice, he wears a plain black t-shirt and dark green shorts.

Outwardly polite and gentle, especially towards non-teammates, he consistently uses formal speech. With his teammates, however, he reveals a more playful and relaxed demeanor. Despite his generally docile appearance, he projects a formidable presence when angered, which makes fellow teammate Mashiro Tsukiyuki wary.

His role as vice-captain centers on maintaining team equilibrium, particularly moderating captain Tōru Takase's passionate clashes with rivals like Masamune Shichigahama from Ao High School. He perceptively recognizes team dynamics and acknowledges the competitive strength of rivals like Ao High, considering them worthy opponents.

His background includes only the maternal connection to Aomori. His development consistently highlights his stabilizing influence within the team structure and his capacity to balance relationships through approachability and necessary authority.