Kenji Kawachi is the central character of the story, initially introduced as a single office worker in his thirties. His life takes a sudden turn following the death of his grandfather, Soichi Kaga. At the funeral, Kenji discovers that his late grandfather had an illegitimate six-year-old daughter named Rin, a fact that shocks the entire family.

In terms of personality, Kenji is depicted as a responsible, kind-hearted, and principled individual, though initially inexperienced with child-rearing. When other family members treat the young Rin as an outcast, openly refusing to adopt her and planning to send her to an orphanage, Kenji is disgusted by their callousness. This situation reveals a strong, decisive core beneath his ordinary office worker exterior, as he makes the impulsive but firm decision to take Rin into his own care.

His primary motivation throughout the story is the welfare and happiness of Rin. He is driven by a sense of duty and genuine affection, choosing to completely alter his comfortable, independent lifestyle to meet her needs. This motivation forces him to navigate the new and daunting challenges of single parenthood, such as managing daycare logistics, adjusting his work schedule, and providing emotional support.

Kenji's role in the narrative is that of the protagonist whose actions drive the plot forward. The story follows his journey from an ordinary bachelor to a dedicated, self-sacrificing father figure. His role is not just that of a caretaker but also a student who learns about the complexities of parenting and the depth of familial bonds. He is the stable anchor in Rin’s life, providing the home and security she lacks.

Key relationships heavily define his character arc. His bond with Rin is the emotional center of the story, evolving from a legal guardian relationship into a deep, mutual connection where she regards him as both a father and a friend. His relationship with his sister, Kazumi, provides familial tension and support; though she initially states a dislike of children and warns him against overworking himself, she eventually grows to adore Rin. A significant relationship also forms with Yukari Nitani, the mother of Rin’s friend Koki, who becomes a crucial source of parenting advice and companionship. Additionally, his confrontation with Rin’s biological mother, Masako Yoshii, reinforces his moral code, as he is disgusted to find she has no maternal feelings for Rin, leading to his final decision to formally adopt the girl.

Kenji undergoes significant development throughout the story. He starts as a man unfamiliar with the daily grind of raising a child, struggling to balance work at an office and the need to pick up Rin from a distant daycare center. His growth is demonstrated by his willingness to sacrifice his career trajectory; he chooses a demotion to a labor position to reduce his fatigue and secure more time with her after work. This change ultimately makes him happier, as he finds joy in playtime with Rin, transforming from a fatigued salaryman into a fulfilled and devoted parent.

While Kenji does not possess any superhuman or exceptional physical abilities, his notable abilities are his practical problem-solving skills and his deep capacity for empathy and selflessness. He methodically addresses the practical hurdles of parenthood, such as rearranging his entire work life, and shows a strong emotional intelligence in gaining Rin’s trust and defending her from social stigma.