Yusuke Azumi, a junior high school student and Norimichi Shimada’s close friend, competes in a high-stakes swimming race orchestrated by Nazuna Oikawa, who promises a date to the winner. Though Yusuke claims victory, he abruptly cancels the rendezvous, souring his bond with Norimichi and exposing his unreliability in matters tied to Nazuna. His uncertainty about her romantic interest—wavering between himself and Norimichi—fuels a rivalry intensified by timeline shifts caused by a magical glass marble. Across alternate realities, Yusuke dismisses Nazuna’s affections with casual derision, revealing his inability to empathize.

When Norimichi rewrites events to secure victory and pursue Nazuna, Yusuke’s jealousy erupts into hostility. He sabotages their attempts to elope, confronting them at critical junctures and violently disrupting their escape by shoving both off a lighthouse balcony. This aggression cements his role as an obstacle driven by envy, clashing with Norimichi’s loyalty to Nazuna and fracturing their friendship.

Yusuke’s rigid adherence to the group’s original plans contrasts Norimichi’s prioritization of Nazuna, deepening the conflict over trust and allegiance. His relentless interference—spotting the pair at a railway crossing, thwarting their flight—echoes his static characterization, devoid of redemption or personal growth. The narrative frames him as a fixed antagonist, his choices propelling the central tensions of the love triangle without delving into his history or motivations beyond his role as a catalyst for conflict.