Koichi Hirose recurs as a supporting character across multiple episodes of the spin-off series. His established personality and dynamic with Rohan Kishibe carry over from *Diamond is Unbreakable*, evolving from an initial antagonistic encounter into a trusted ally and frequent companion for investigating supernatural phenomena. In "At a Confessional," Koichi accompanies Rohan to a church. They encounter a man possessed by a malevolent spirit within a confessional booth. Koichi observes Rohan deploying Heaven's Door to read the man's memories, uncovering the spirit's origin linked to a historical execution site. Koichi provides a grounding presence, interjecting to prevent Rohan from crossing ethical boundaries during their inquiry. For "Mutsu-kabe Hill," Koichi joins Rohan to investigate rumors of a cursed location causing disappearances. While specific actions by Koichi are less documented, his participation underscores his recurring role as Rohan's investigative partner confronting paranormal threats tied to specific locales. During "Poaching Seashore," Koichi assists Rohan in confronting illegal fishermen at a beach plagued by supernatural occurrences tied to marine exploitation. He witnesses Rohan's encounter with their ally Tonio Trussardi and observes how the exploitation disrupts natural and supernatural balances. Koichi helps document the environmental and ethical violations, reinforcing his role as a moral counterbalance to Rohan's sometimes extreme methods. In "Millionaire Village," Koichi participates in Rohan's investigation of a wealthy, secluded community rumored to be cursed. He aids in uncovering the village's dark secrets involving distorted reality and temporal anomalies affecting residents. Koichi's perceptiveness contributes to resolving the central mystery as he identifies patterns within the supernatural phenomena. Across these episodes, Koichi retains his core characteristics: politeness, inherent moral courage, and perceptiveness. He consistently acts as Rohan's ethical anchor, questioning the manga artist's intrusive methods while remaining a reliable ally. Their dynamic reflects mutual respect tempered by Koichi's willingness to challenge Rohan's excesses, particularly regarding the use of Heaven's Door on unsuspecting individuals. No significant evolution or deviation from his established personality in *Diamond is Unbreakable* is depicted within the spin-off.