Reiji Kokonoe became guardian and first cousin once removed to Rin Kokonoe after her mother Aki Kokonoe's death. His early life held deep trauma: an abusive father prone to screaming and a neglectful mother, culminating in his parents' deaths in a traffic accident after he isolated himself due to domestic violence. Aki and Rin took him in, though he remained cold and distrustful initially, suspecting Aki sought marriage using Rin as an excuse like his own mother.

He developed a profound bond with Rin and fell in love with Aki despite their thirteen-year age gap and familial tie. Aki's refusal to abort Rin at her biological father's request and her strength as a single mother deeply affected him, making him wish his own mother had been like her. Their relationship turned romantic, and Reiji sought work to support Aki after her lung cancer diagnosis. Her death shattered him emotionally; he assumed Rin's guardianship, vowing to shield her from adult betrayal.

As Rin's guardian, Reiji exhibits fierce protectiveness, especially concerning her interactions with teacher Daisuke Aoki. Jealousy over Rin's affection for Aoki fuels his initial hostility and distrust. Reiji's unresolved grief for Aki manifests in an unhealthy obsession with Rin, including borderline sexual advances and grooming behaviors aimed at molding her into Aki's replacement. This inflicts significant emotional distress on Rin, triggering her fainting spells and withdrawal. Aoki's interventions eventually help Reiji reform, learning impulse control and gradually trusting Aoki, though he struggles to admit this growth.

A pivotal moment occurs when Rin's biological father, Roku Kamihara, reappears seeking contact. Reiji reacts with panic and resistance, fearing Rin's loss and reflecting his lingering insecurities. Rin confronts the situation with composure, decisively rejecting Roku and reaffirming her loyalty to Reiji, highlighting their complex dynamic: Reiji's fragility against Rin's resilience.

Reiji's character arc concludes with significant redemption. He overcomes his fixation on Aki and develops genuine feelings for Mimi Usa, Rin's classmate who shares his history of emotional vulnerability. Acknowledging their parallel wounds, he encourages her to avoid succumbing to anger and hatred. In the final chapter, he commits to marrying Mimi once she reaches adulthood, symbolizing his capacity for healthier relationships and a stable future, marking his evolution from a figure consumed by past trauma to one capable of forward-looking emotional maturity.