Natsuo Fujii is a black-haired, olive-green-eyed high school student and aspiring novelist who habitually writes on his school rooftop. This routine leads him to meet an English teacher who inspires him and becomes his initial romantic interest. Presuming his feelings unreturned, he attends a mixer, encountering the reserved Rui. Leaving together, she suggests they engage in sexual activity for experience, which he accepts during a moment of vulnerability.

His widowed father soon announces remarriage. At the family introduction, Natsuo discovers his father's new partner is the mother of both his admired teacher and Rui, causing him to faint. This creates immediate tension within the new blended family.

His current polished appearance – straightened hair and contact lenses – contrasts sharply with his junior high look of naturally curly hair and glasses, a change suggested by a friend and known to few. Kind, warm, and approachable, he unintentionally attracts romantic interest while often remaining oblivious. Prone to indecisiveness, this frequently leads to misunderstandings and unintended consequences. Writing serves as his creative outlet and emotional coping mechanism, sometimes distracting him during class. He habitually suppresses his emotions, particularly after a traumatic breakup, hindering his awareness of how his actions affect others.

His childhood was marked by the profound loss of his mother to illness. Vowing not to cry at her funeral to appear strong, his father reassured him expressing grief was natural. He coped by retreating into his mother's books, forging a deep connection to literature that fueled his aspiration to become a novelist. His goal is to move readers as books comforted him, leading him to seek mentorship, win a literary award, and demonstrate dedication to the craft.

His relationships are complex. He develops a bond with his step-sister Hina on the school rooftop, sensing her hidden sadness and offering support. Discovering her involvement with a married man and navigating their parents' marriage, they begin a secret relationship. Exposure forces her sudden transfer and ends contact. Later, he physically shields her from a stalker's attack. Ultimately, they marry after dramatic events involving her sacrifice to protect Rui and a subsequent coma.

Concurrently, he forms a connection with his other step-sister, Rui. Their mixer encounter evolves into cohabitation after their parents' marriage. She develops genuine feelings, though his early focus is on Hina. After his breakup with Hina, they begin a long-term relationship extending into university. Long-distance separation during her studies abroad and his writer's block strain their connection, leading to a temporary breakup. They reconcile and plan to marry, disrupted by Hina's injury. Though their romance concludes, they have a daughter and maintain cooperative co-parenting.

At university, joining a drama club reveals his writing talent, leading to scriptwriting opportunities. He faces professional challenges like creative block and dangerous situations stemming from his indiscriminate helpfulness. His career advances upon winning a prestigious literary award, though he objects to his publisher exploiting his personal trauma.

Significant relationships include his supportive friendship with Fumiya Kurimoto, platonic bonds with literature club members Miu Ashihara and Momo Kashiwabara (the latter developing unrequited feelings), and mentorship under Reiji Kiriya, a teacher and published author guiding his writing.

His later life involves living with Hina, Rui, and their daughter in a shared household while continuing his professional writing career within this complex family structure.