Hiromu Shinbo is a supporting character and a close friend of the protagonist, Hideki Motosuwa. He attends the same preparatory school, Seki cram school, where he is a classmate of Hideki's. In the anime adaptation, Shinbo lives in the same apartment building as Hideki and is his next-door neighbor, which makes him an easily accessible confidant.

Shinbo is a young man of average to tall height, standing around 180 centimeters, with black hair and eyes that are depicted as either gray or green in various sources. He possesses a relaxed and amiable personality, often appearing more laid-back and worldly than his friend Hideki. Shinbo is consistently portrayed as kind and friendly, a trait that is noted by other characters. This goodness is evident early on when he helps the newly arrived Hideki carry his belongings to his apartment without expecting anything in return. His calm and helpful demeanor makes him a reliable figure, particularly in matters that confuse Hideki.

A significant part of Shinbo's role in the narrative is to act as a guide for Hideki, who is largely unfamiliar with the personal computers, known as persocoms, that are a common part of everyday life. Shinbo is the owner of a small, mobile persocom named Sumomo, who has a cheerful and energetic personality that he likely programmed. In the story, Shinbo often uses Sumomo to assist Hideki, whether it is by carrying out searches, relaying messages, or attempting to analyze the mysterious persocom Chi that Hideki finds. When Sumomo fails to analyze Chi, Shinbo is the one who introduces Hideki to a young technological prodigy named Minoru Kokobunji, who might have more advanced capabilities. Through these interactions, Shinbo fulfills the function of a bridge between the technologically inept Hideki and the more complex world of persocoms.

Shinbo's character is also central to a significant secondary romance. He becomes romantically involved with his and Hideki's teacher, Takako Shimizu. Their relationship begins when he finds her alone in a park late at night, having been accidentally locked out of her house by her husband, who was too absorbed in his work with a persocom to notice. This relationship deepens over the course of the series, leading to a point where Shinbo decides to elope with Takako. His determination profoundly affects his own life and culminates in a more stable resolution for the couple. In a gesture connected to this development, after he and Takako begin their life together, Shinbo gives his persocom Sumomo to Hideki, as Takako is still hesitant about living with persocoms due to her past experiences.

Hiromu Shinbo’s primary motivation appears to be the pursuit of genuine human connection. He is a supportive and steadfast friend to Hideki, offering advice and assistance without condescension. Simultaneously, he actively seeks and fights for a romantic relationship with Takako, demonstrating a willingness to defy social conventions to achieve personal happiness. His character arc moves from that of a helpful and knowledgeable student and neighbor to a more assertive individual who makes life-altering decisions for love. His notable abilities are not physical or combative, but rather social and technical. He possesses a functional understanding of persocoms, enough to operate and maintain his own unit, Sumomo, and to assist Hideki with basic tasks. More importantly, his kindness, patience, and perceptiveness are his defining traits, allowing him to form strong bonds with Hideki and eventually win the heart of Takako Shimizu.