Reiji Kokonoe serves as both guardian and first cousin once removed to Rin Kokonoe. His traumatic childhood featured abuse from an alcoholic father and maternal neglect, forging profound resentment toward his parents. Their fatal car accident elicited no grief from him. He relocated to Tokyo afterward, joining his cousin Aki Kokonoe and her daughter Rin. Though initially wary of Aki, Reiji cultivated romantic feelings for her, resulting in a relationship that defied familial connections and age disparity. They married, but Aki succumbed to lung cancer, leaving Reiji as Rin’s legal guardian.

As guardian, Reiji displays fierce protectiveness and jealousy, notably directed at Rin’s homeroom teacher Daisuke Aoki because of Rin’s fondness for him. This rivalry originates in Reiji’s terror of losing Rin and his lingering attachment to Aki, whom Rin physically mirrors. His protectiveness occasionally borders on possessiveness, with undercurrents of grooming behavior intended to position Rin as his future wife. Yet his attempts to dominate Rin’s life conflict with his authentic concern for her welfare and his emotional fragility. Gradually, Reiji’s interactions with Aoki shift from open antagonism to cautious trust, recognizing Aoki’s beneficial role in Rin’s life.

Reiji’s evolution involves facing his past and curbing his jealousy. His early immaturity and struggle with adult duties starkly contrast Rin’s prematurely cultivated maturity. A turning point unfolds when Rin’s biological father seeks custody; Reiji reacts with panic while Rin addresses the crisis with composure, underscoring their role reversal. This incident, coupled with Aoki’s support, sparks Reiji’s incremental growth. His connection with Mimi Usa, Rin’s friend, grows consequential. Mimi nurses a crush on him and mirrors his emotional vulnerability, deepening their bond. By the manga’s end, Reiji abandons his fixation on Aki, nurturing mutual affection for Mimi and envisioning a shared future upon her adulthood.