Rintaro Kira endured a childhood scarred by parental abuse, suffering physical violence from his biological father and emotional neglect from his mother. Following their divorce, his mother remarried, but his stepfather also became abusive. This cycle resulted in his placement in an orphanage around age nine, where he isolated himself and avoided others. There, he met Hinako Nakayama. When other children bullied him, he nearly retaliated violently, but Hinako intervened. This pivotal moment made him resolve never to become like his father, choosing instead to use his strength for protection. He viewed Hinako as his savior and later taught her self-defense. Before leaving the orphanage after six months to return to his family, he gave her a string bracelet as a token of friendship. His home situation remained abusive, leading him to move out independently during high school, supporting himself by working as a café waiter to cover rent and expenses.

He attended Shishiku Academy, an all-boys school for delinquents. His exceptional martial arts skills, particularly in boxing, established him as the top fighter among second-year students. Despite this reputation, he maintained a solitary, reserved, calm, and stoic demeanor, avoiding unnecessary fights. His life shifted when he encountered Hinako again, disguised as her twin brother, Hikaru Onigashima. He recognized her through the bracelet. Discovering her secret, he vowed to protect it while navigating complex feelings, including jealousy during her interactions with other male students. He also reconnected with his stepbrother, Nozomi, who initially resented him. Despite their strained relationship, he showed underlying concern for Nozomi, especially after their mother abandoned the family, leaving Nozomi vulnerable to their stepfather's abuse. He intervened when yakuza threatened his family, prioritizing Nozomi's safety over past grievances.

His character deepened through his evolving relationship with Hinako. After she revealed her identity to him, his protective instincts intensified, shifting from brotherly care to romantic feelings. He expressed discomfort with being called "oniichan" and confessed his desire for a deeper connection. This caused temporary friction, but they reconciled, with him accepting her choices while remaining her protector. He also worked towards reconciling with Nozomi, notably when they fought off yakuza targeting their family. He pursued a career as a pastry chef, committing to this path in his later school years. A setback occurred on Valentine's Day when he injured his arm defending against yakuza, jeopardizing his culinary exams. Supported by Hinako and Nozomi, he recovered, passed his exams, and secured a position at a renowned restaurant. He later invited Hinako to live with him after graduation, symbolizing their shared future. His story concluded with them sharing a kiss under cherry blossom trees, marking his growth from a traumatized orphan to a self-reliant individual dedicated to protection and love.