Koutarou Miyao serves as Haruo Yaguchi's classmate and close friend through middle school and high school, characterized by brown hair, blue eyes, and a typical adolescent look. He functions as a steadfast brother figure to Haruo, consistently providing support and counsel on Haruo's complex relationships and personal struggles. Miyao’s observant nature enables him to detect subtle emotional shifts in others, such as Koharu Hidaka's unspoken feelings for Haruo and their effect on her mood.

He exhibits a protective streak toward Haruo, stepping in to shield him from confrontations involving Akira Ono and Koharu Hidaka and displaying visible discomfort during interactions tied to their love triangle. Though Miyao develops romantic feelings for Akira himself, he deliberately steps back from pursuing her to avoid disrupting Haruo's connection with her. His loyalty manifests in covering for Haruo during tense moments.

Beyond this confidant role, Miyao regularly socializes with Haruo and their classmate Genta Doi, forming a recurring trio that frequents local arcades—a dynamic sustained into their high school years. As an adult aged 28 in the spin-off *Hi Score Girl DASH*, Miyao reappears, confirming his continued presence in the narrative beyond the original series.