Yusuke Azumi is a male student in Moshimo and one of Norimichi Shimada's friends. He competes against Norimichi in a swimming pool race organized by Nazuna Oikawa to decide her summer festival date invitation. Yusuke wins the initial race, prompting Nazuna to ask him out. Later, he evades this commitment by instructing Norimichi to cancel the date under the guise of receiving medical treatment for a minor injury.

This avoidance creates tension when Yusuke witnesses Nazuna spending time with Norimichi. His reaction escalates into a physical altercation with Norimichi near a lighthouse, culminating in Yusuke pushing them both off the structure. This fall triggers a reset of events through a mysterious glass marble.

In altered timelines caused by Norimichi's interventions, Yusuke's behavior shifts. When Norimichi wins the swimming race and becomes Nazuna's date, Yusuke interprets their interactions as betrayal. His resentment fuels persistent anger and hostility, highlighting his jealousy and sense of entitlement toward Nazuna's attention.

Yusuke's background is minimally detailed: he lives with his father, a doctor, which facilitates his medical excuse. His actions primarily serve as catalysts for conflict and timeline resets without substantive character development. His role diminishes as Norimichi and Nazuna repeatedly attempt to evade peers and family. Yusuke's unresolved animosity and the consequences of his actions remain unaddressed by the narrative's conclusion.