Hiroshi Nagatsuma is a colleague of Souta Tawara, the protagonist, working together at the same web design company in Tokyo. He is a young man with brown eyes and brown hair that is cut to ear length. His personality is notably quirky and eccentric, which occasionally makes him the subject of gossip among other coworkers who view his different behavior unfavorably. Despite this, Souta has always stood up for him and admired his unique qualities, a kindness that Hiroshi has never forgotten.

Motivation and role in the story become particularly significant when Souta decides to leave his job in Tokyo and return to his rural hometown in Kagawa to care for a strange child named Poco. Upon learning of Souta's plan to quit the team, Hiroshi initially becomes angry and upset, feeling abandoned by the colleague he respects. His suspicions about the situation grow when he later visits Kagawa and is the first to notice that Poco is not an ordinary child, spotting the boy's tail and animal-like ears. This makes Hiroshi a central figure of tension, as he comes close to uncovering the secret that Poco is actually a magical tanuki.

However, this conflict drives Hiroshi's most meaningful development. He is also dealing with his own personal pain, specifically a broken heart from an unrequited love for a married woman. As he spends more time with Souta and Poco, he is reminded of the acceptance Souta showed him when they first met. In a key moment of emotional growth, Hiroshi overcomes his suspicion and denial of the supernatural, choosing to wholeheartedly accept Poco regardless of what he might be. He famously declares that he and Poco will be friends forever, demonstrating his capacity for unconditional acceptance. This arc reinforces one of the series' core themes about embracing differences and the enduring nature of true friendship.

Hiroshi also serves a practical role in the narrative by pushing Souta to confront his past. Understanding that Souta is grieving his late father, Hiroshi encourages him to make udon noodles, the dish his father was famous for, as a form of comfort food for his broken heart. Through this act, Hiroshi helps Souta begin the process of healing old regrets and reconnecting with his heritage. In terms of abilities, Hiroshi does not possess any supernatural traits; his most notable skill is his keen observation, which allows him to be the first character to notice Poco's inhuman features. His persistence and forward-pushing nature make him a catalyst for much of the drama and emotional resolution in the latter part of the story.