Kōtarō Miyao is Haruo Yaguchi's classmate and close friend throughout middle school and high school. His age spans 12 to 16 during the main events, later appearing at 28 in *Hi Score Girl Dash*, though his adult life specifics are not detailed.

Miyao adopts a brotherly role toward Haruo, consistently offering emotional support and guidance, particularly concerning Haruo's romantic difficulties with Ono Akira and Hidaka Koharu. He perceptively identifies Hidaka's feelings for Haruo and notices shifts in her emotions, yet avoids direct involvement. Miyao develops his own romantic feelings for Ono Akira but deliberately suppresses them to preserve Haruo's relationship with her, prioritizing their friendship.

His interactions with Hidaka Koharu add emotional complexity. Miyao exhibits discomfort during situations involving Haruo with both Hidaka and Ono, hinting at possible but ambiguous feelings for Hidaka. He occasionally encourages Hidaka's gaming interests, reflecting his supportive nature.

Miyao's personality blends deep loyalty with emotional restraint. He actively protects Haruo from confrontations involving Ono and Hidaka. Internal conflicts surface subtly through ambiguous expressions or remarks straddling jest and sincerity, such as jokingly threatening to kiss Haruo during an argument about Ono—his expression hinting at seriousness. Another instance sees him refer to Haruo possessively as "My Haruo," prompting Haruo's questioning reaction. These moments suggest unresolved emotions beneath his supportive exterior.

His family background includes unnamed parents, older siblings, and younger siblings, though their specific identities or influence remain unexplored.