Kosuke Sakaki attended Sendai Josei High School, competing on the track and field team alongside Kakeru Kurahara. Sakaki resented Kakeru's exceptional talent and the coach's overt favoritism, which involved publicly praising Kakeru while harshly criticizing and punishing other team members for average performances, fostering significant animosity.

A pivotal incident occurred when Kakeru physically assaulted their coach after prolonged mistreatment. The school responded by withdrawing the entire team from competitions for a year, nullifying their collective efforts. Sakaki and other teammates blamed Kakeru for destroying their athletic opportunities, solidifying Sakaki's deep-seated grudge.

Later, as university students, Sakaki encountered Kakeru unexpectedly during a training run. Sakaki's lingering hostility surfaced in his critical and accusatory questioning about Kakeru's new Kansei University teammates. His sarcastic remark about Kakeru making friends despite their history conveyed judgment, implying Kakeru did not deserve companionship after the consequences of his past actions.

Sakaki's narrative function centers on embodying Kakeru's unresolved high school trauma. He serves as a persistent reminder of Kakeru's past failures in teamwork and social relationships, forcing Kakeru to confront lingering guilt and isolation.

During the Hakone Ekiden marathon, Sakaki competed against Kansei University. Observing Kakeru run wholeheartedly for his team's success, not just individual glory, marked a turning point. Witnessing this genuine display of teamwork and camaraderie led Sakaki to finally acknowledge Kakeru's growth and personal change. This acceptance allowed him to move past his resentment, symbolizing the resolution of their conflict.