Yū Mashiro begins his journey as a Doushisha University first-year and rugby scrum half, distinguished by a delicate frame and notably frail physique. His medium-length blue hair features an ahoge and precise v-shaped bangs, framing green eyes. Malnutrition and stress trigger gastric ulcers and anemia, culminating in a training collapse that hospitalizes him and ends his playing days. Inspired by Natsusa Yuzuki—who shifted to management after his own injury—Mashiro embraces the team manager role.

Shy and gentle, Mashiro flusters easily when praised. He idolizes Natsusa, seeking guidance on senior interactions and striving to mirror his rugby philosophy despite physical constraints. Teammates deeply respect his kindhearted nature, rarely opposing him.

Earlier, Mashiro watched Natsusa’s rugby videos before his return. Hospitalized after collapsing, he voices frustration over his limitations to Natsusa but fiercely reaffirms his rugby passion. This exchange fuels his resolve to stay in the sport. The next day, he announces his managerial commitment to the team.

His name reflects his essence: "Yū" (優) signifies "gentleness, excellence," while "Mashiro" (真白) blends "genuine" and "white."