Rumi Shingūji, one of the fraternal Jingūji twins, lives next door to childhood friend Jun Shirosaki. She possesses a tomboyish personality, actively participates in the basketball club, and sports a short hairstyle, distinguishing her from her sister Naori. Beneath her sporty exterior, Rumi harbors deep romantic feelings for Jun.

During middle school, Rumi impulsively initiated a romantic relationship with Jun, convincing him to date her. Overcome by immediate guilt over interfering with Naori's affection for Jun, she insisted their relationship stay secret. As months passed, this inner conflict intensified, leading Rumi to break up with Jun. She asserted Naori was better suited for him and pressured Jun to pursue her sister instead.

After the breakup, Rumi wrestled with regret and jealousy, especially witnessing Jun's growing closeness with Naori. In a charged encounter within Jun's bedroom, they nearly kissed before Rumi rejected him again, reiterating her demand that he date Naori. When Jun and Naori eventually began dating, Rumi feigned happiness to avoid burdening her sister with guilt, masking her own deep conflict and unhappiness. Her friend Reira Asano later urged her to prioritize her feelings, highlighting Rumi's ongoing struggle between her love for Jun, loyalty to Naori, and tendency for self-sacrifice.

Rumi's actions consistently reveal a pattern of sacrificing her own happiness for her sister's perceived benefit while grappling with jealousy and regret over the consequences. This complex interplay of impulsiveness, guilt, and unresolved emotions shapes her relationships and personal conflicts within the enduring love triangle.