Yuuma Otomine is Ajimu Yasuna's former boyfriend, their relationship ending roughly a year before the main events when he suddenly left her. With help from someone named Koharu, he deceived Ajimu into believing he had died, using this fabricated death as his reason for vanishing from their town. His true motive was moving to London to fully dedicate himself to his art education.

After an unspecified time abroad, he returned to his hometown, staying at the Izumiya family's inn. Another resident there, Moe, developed romantic feelings for him during his stay. His reappearance complicated matters for Ajimu and Hirosuke, as both Yuuma and Hirosuke held feelings for Ajimu. Despite this romantic rivalry with Hirosuke, Yuuma remained friendly and approachable. Their interactions involved a lighthearted, non-confrontational competition for Ajimu's affections.

His return was driven by a desire to rekindle his past relationship with Ajimu, indicating unresolved feelings. It remains unclear whether he ever explicitly defined the nature of their prior relationship to Ajimu. His presence primarily acted as an obstacle to the developing connection between Ajimu and Hirosuke, though the rivalry consistently lacked overt hostility.