Koutarou Miyao is Haruo Yaguchi's classmate and close friend throughout middle school and high school. He possesses brown hair and blue eyes and functions as a brother figure to Haruo, consistently offering support and advice, especially concerning Haruo's romantic involvements with Akira Ono and Koharu Hidaka. Miyao demonstrates perceptiveness by recognizing Hidaka's feelings for Haruo early and noting shifts in her emotional state.

His loyalty is evident when, despite developing feelings for Ono, he deliberately steps aside to avoid disrupting her relationship with Haruo. Miyao displays complex emotions toward Haruo, including moments implying non-platonic feelings, such as jokingly threatening to kiss Haruo to force a confession about Ono, though his expression hints at seriousness. He also refers to Haruo possessively as "My Haruo," which Haruo notices. Miyao appears uneasy around the love triangle involving Ono and Hidaka, further indicating his awareness of their complicated dynamics.

In the sequel series *Hi Score Girl Dash*, set years later, the 28-year-old Miyao works in a corporate setting. He maintains his friendship with Koharu Hidaka, now a middle school teacher. He attempts to mitigate her loneliness by arranging introductions with colleagues, which she rejects. His concern for her well-being persists despite her disinterest in romance. Across both series, Miyao acts as a stabilizing presence within his friend group, prioritizing others' happiness over his own romantic interests while navigating interpersonal tensions with observational acuity.