Oosu Seri shares a close childhood bond with Yagi Takeru, having grown up alongside him after he joined the Tōma household following his parents' death. As the granddaughter of Tōma Rokunosuke, the family patriarch who raised Takeru, Seri maintains constant proximity to him within their shared home. Her martial arts training reflects a disciplined background in physical combat.

During high school, Seri displays distinctive traits including a hair antenna and snaggletooth, often accessorized with a headband. Her routine interactions involve Takeru, their mutual friend Yamato Takeshi, and the Shiratori sisters, Kotono and Asuka.

When an earthquake transports her to an unfamiliar world alongside Takeru and Asuka, Seri is classified as a "Person in a Strange World." She flees the ruined school with her companions, confronting their vanished town before encountering cloaked figures who clarify their displacement.