Jun Koshinae, a classmate and friend of Ichitaka Seto, emerges during a school trip to Kyoto, where he is initially partnered with Iori Yoshizuki. His apparent romantic interest in Iori sparks tension, casting him as a rival in Ichitaka’s eyes. This dynamic shifts when Jun unexpectedly reveals his homosexuality and secret affection for Higemi-sensei, their teacher. Seeking proximity to Higemi-sensei—paired with Ichitaka during the trip—Jun proposes a partner trade, inadvertently aiding Ichitaka’s budding connection with Iori.

Post-trip, Jun evolves into a trusted confidant for Ichitaka, supplementing Teratani’s advice with grounded emotional support during moments of doubt. Though reserved and conflict-averse, Jun’s latent martial arts skills surface decisively to protect friends in peril, contrasting his typically composed nature.

Throughout the narrative, Jun remains a stabilizing anchor amid Ichitaka’s romantic turbulence. His unrequited feelings for Higemi-sensei linger unresolved, underscoring personal struggles intertwined with his loyalty to friendships. His portrayal stays narrowly focused on these dynamics, with no expanded exploration in other adaptations.