Naoto Watari is a sixteen-year-old high school sophomore and the central protagonist of the story. Two years prior to the events of the series, his parents died, leaving him and his younger sister Suzushiro to be passed among relatives. They eventually settle in a Tokyo suburb with their reclusive aunt Tamayo, and Naoto takes on the role of primary caretaker for his sister. His world revolves entirely around Suzushiro's well-being; he skips club activities and social opportunities to spend time with her, leading classmates to label him a siscon, a label he ignores as long as his sister is happy. This fierce protectiveness is both a source of strength and a cage, as he suppresses his own desires to maintain a stable, predictable life.

Naoto's personality is defined by responsibility, diligence, and a cautious, often guarded nature. He is a member of his school's Beautification Committee and harbors a quiet crush on fellow committee member Yukari Ishihara, whom he sees as an ideal, normal romantic prospect. However, his carefully constructed routine is shattered when Satsuki Tachibana, his childhood friend, transfers into his class. Years earlier, Satsuki destroyed a family vegetable garden in an incident that traumatized Naoto and caused him to develop deep resentment toward her. He was close to her before that event, but her sudden departure left unresolved pain. Her return forces him to confront buried memories and navigate a volatile relationship that oscillates between hostility and lingering affection.

Naoto's motivations are dual: he strives to protect his sister and preserve their fragile domestic stability, while also grappling with the need to process his past trauma. Satsuki's provocative behavior, including a sudden kiss that shocks everyone, catalyzes the collapse of his careful defenses. His role in the story is that of a reluctant hero drawn into a complex web of romantic and emotional entanglements. Beyond the central conflict with Satsuki, his relationship with Yukari Ishihara develops as a potential normal romance, adding a love triangle dynamic. His friendship with classmate Shigenobu Tokui provides comic relief and a sounding board, though Naoto often ignores Tokui's advice.

Over the course of the narrative, Naoto evolves from a hyper-focused, self-sacrificing guardian to someone who begins to acknowledge his own feelings and desires. He learns to balance his duties to his sister with personal growth, gradually letting go of his resentment toward Satsuki as the truth behind her past actions is revealed. His notable abilities are not supernatural; rather, they lie in his resilience, his capacity for dedication, and his eventual willingness to confront emotional vulnerability. He is depicted with short black hair and brown eyes, and he frequently wears his school uniform. His development is marked by the increasing complexity of his relationships with the girls in his life, as he moves from a state of denial toward acceptance of his own heart.