Hirotoshi is a character introduced in the later arcs of the story, specifically during the modern era narrative. He is a thirty-year-old man who lives with his father. At the time of his introduction, Hirotoshi is unemployed and spends almost all of his time at home, dedicating himself to hobbies such as playing video games and reading manga. His room is often described as being unkempt, and he shows little motivation to find a job or change his lifestyle. A significant and controversial aspect of his personality is his deep fascination with the little sister character archetype found in otaku culture, and he possesses a collection of manga that focus on this theme.

Hirotoshis life changes dramatically when his father remarries a woman named Miyabi, who has a ten-year-old daughter named Mimori. Upon meeting Mimori, Hirotoshi is immediately captivated by her, seeing the arrival of a real-life little sister as the fulfillment of a long-held dream. Although his initial interest in the young girl has overtones that the narrative presents as troubling and obsessive, his behavior evolves. He becomes genuinely devoted to Mimoris well-being, paying close attention to her and noticing details that others, including her own mother, overlook. For instance, he observes that Mimori never smiles and seems deeply unhappy. In an effort to connect with her and bring her joy, he offers to teach her a special skill of his: making a perfectly polished, shiny mud ball.

Hirotoshis primary role in the story is as the protective stepbrother who is also a witness. He attends a parents day event at Mimoris school when her own parents are absent. On that day, he witnesses Mimori fall from the school rooftop. The shock of this event is compounded when she stands up unharmed but with a completely different, unnaturally cheerful personality. While her mother and stepfather accept the change, Hirotoshi becomes convinced that this cheerful girl is an imposter, a replacement for the real Mimori he had come to care for. He spends an entire year surreptitiously watching her, hoping to find evidence or a way to bring the original Mimori back.

His key relationship is with his step-sister, Mimori, and his entire motivation is centered on her. Despite the questionable nature of his initial obsession, Hirotoshis actions are driven by a desire to save the real girl he believes is lost. He is willing to put himself in danger, even fighting against the Nokker that has taken her form, to uncover the truth and rescue her spirit. His determination is fueled by his belief that Mimoris neglectful mother never truly cared for her, and that someone must fight for her happiness. His relationship with his father is distant, defined by his fathers disappointment in his lack of ambition.

Over the course of this arc, Hirotoshi shows signs of development. His experiences force him to move beyond his self-centered, reclusive life. He begins taking on household responsibilities, such as cleaning and cooking for Mimori. His devotion to her transforms from a selfish fantasy into a selfless, if still unconventional, form of love and heroism. He becomes a character whose unsavory traits exist in tension with his genuine capacity for care and sacrifice.

He does not possess any supernatural abilities. However, he considers the ability to create a perfectly polished and shiny mud ball to be his special skill, a talent he tries to pass on to Mimori to help her gain confidence and make friends.