Mashiro Tsukiyuki attends Hakumei University Affiliated High School (Shiro High) as a first-year student and competes on the boys' rhythmic gymnastics team. He shares a junior high background in Miyagi Prefecture with Ryoya Misato, whose gymnastics performance originally inspired him to pursue the sport.

Frequent childhood relocations across Japan stemmed from his father's video cameraman profession. After settling in Miyagi, Mashiro discovered rhythmic gymnastics and rapidly displayed exceptional natural talent, securing victory in a junior competition despite previously abandoning other sports due to disinterest.

His personality blends naïveté, capriciousness, and airheaded tendencies. Mashiro relishes playful interactions and dives intensely into enjoyable activities, often neglecting responsibilities. Though approachable and socially driven, he operates at an unhurried personal pace that can seem detached.

Contrasting this whimsy, Mashiro exhibits fierce concentration during gymnastics, describing a trance-like state where surroundings turn "completely white, like a blank canvas." He nurtures intense rivalries with admired figures like Misato and Shotaro Futaba—the latter initially dismissed but later sparking Mashiro's curiosity through visible growth.

Physically, Mashiro stands out with stark white hair and pink eyes. He typically wears a fully zipped tracksuit or blue hoodie, while practice outfits feature an oversized white-and-blue shirt bearing a prominent "46" print, paired with white shorts.

Across his journey, Mashiro forms competitive bonds and learns to harness his innate talent within team dynamics. Despite ongoing relocations with his father, gymnastics anchors him with stability and purpose.