Hiroshi, often referred to as Hiroshi-kun, is an anatomical figure or mannequin that serves as the most treasured possession and closest companion of the series' protagonist, Sunako Nakahara. As an inanimate object, Hiroshi does not have a personal history, but his significance to Sunako is rooted in the circumstances of his discovery. Sunako found him discarded in a pile of rubbish shortly after she was harshly rejected and called ugly by her first crush. Feeling abandoned by the world herself, she formed an immediate and deep bond with the mannequin, seeing in him a kindred spirit.

In a narrative sense, Hiroshi's personality is an extension of how Sunako perceives and interacts with him. While he cannot speak or move on his own, Sunako treats him as if he is alive, confiding in him about her innermost feelings, frustrations, and observations regarding the four handsome boys with whom she lives. He is not merely a doll but a genuine friend and confidant to her. This treatment is so pronounced that in the anime adaptation, Hiroshi takes on a meta-narrative role, being depicted as the narrator for the opening of each episode and even participating in the dance during the ending credits, as if he were a living member of the cast.

Hiroshi's primary role within the story is to act as a crucial emotional anchor for Sunako and a physical symbol of her deep-seated trauma and isolation. He represents a safe, non-judgmental presence in her life, a stark contrast to the "creatures of light," or beautiful people, who cause her physical distress. Because of his importance, Hiroshi often becomes a catalyst for plot events. On several occasions, he is taken or kidnapped by other characters to be used as a bargaining chip to manipulate Sunako. For instance, a group of obsessive fans of one of the male leads, Kyohei, once stole Hiroshi to force a meeting with him, leading Sunako to risk her own safety to rescue her beloved friend.

The most significant relationship in Hiroshi's context is, of course, with Sunako. She takes meticulous care of him, as well as her other anatomical friends like Josephine and Akira-kun, and is known to handle his removable fake organs with a sense of normalcy. Initially, the four boys—Kyohei, Takenaga, Yuki, and Ranmaru—are dismissive or unnerved by Sunako's intense attachment to a mannequin. However, as the series progresses, they come to understand and accept the bond, eventually treating Hiroshi with a similar level of respect, all the while acknowledging that he is not alive. This acceptance marks a key development in their relationship with Sunako.

While Hiroshi himself undergoes no personal development, his function in the story highlights Sunako's own growth. The fact that the handsome young men in her life come to accept her unusual friend demonstrates her gradual influence on them and their growing understanding of her unique worldview. Hiroshi has no special abilities in the conventional sense, but his most notable "ability" is his symbolic power: he is a steadfast, unchanging source of comfort for Sunako, embodying her loyalty to the darkness and the discarded, and his role as an animated narrator and dancer in the anime grants him a unique, whimsical presence beyond that of a simple prop. His false organs also have a well-known habit of falling out, which is played for comedic effect.