Kojima is a shy, somewhat awkward high school student who dated Kanon Miyamori. He genuinely cared for her from the start and gave her a ring during their relationship. Misunderstandings and Kanon's insecurities, particularly her perception of Kojima growing distant, led to their breakup. Kanon later discarded the ring into the ocean, making it central to unfolding events.

Initially appearing distant through Kanon's perspective, Kojima later actively communicates his feelings. He explains he misinterpreted Kanon's attempts to create distance, believing she no longer wanted closeness. He expresses hope to see Kanon smile again after resolving their conflict.

Kojima becomes a target for Sedna's forces. Two soldiers capture and nearly kill him just as Kanon tries to reconcile. His presence affects others; classmate Oshima harbors strong romantic feelings for him, viewing Kanon as a rival. Oshima feels inadequate because Kojima frequently talked about Kanon during their own interactions.

Kojima's experiences highlight themes of miscommunication and the consequences of unexpressed emotions, underscoring the narrative's focus on how suppressed sorrow and misunderstanding foster personal and collective darkness.