Kōtarō Miyao serves as Haruo Yaguchi's classmate and close friend throughout their middle school and high school years, sharing an age range of 12 to 16 during the series' events. He possesses brown hair and blue eyes, and his family includes unnamed parents alongside older and younger siblings. Miyao adopts a brotherly role toward Haruo, providing steadfast support and guidance, especially concerning Haruo's romantic pursuits. He shields Haruo during clashes involving Akira Ono and Koharu Hidaka, demonstrating his protective nature. His observant traits allow him to detect Hidaka's romantic feelings for Haruo and her subtle mood fluctuations. Miyao once develops feelings for Ono but withdraws to avoid disrupting Haruo's connection with her.

Miyao's exchanges with Haruo carry layered emotional undercurrents. He once jokingly threatens to kiss Haruo to coerce an explanation about Ono, though his expression implies sincerity. Miyao also refers to Haruo possessively as "my Haruo," eliciting confusion from Haruo, and exhibits unease during the love triangle involving Haruo, Ono, and Hidaka. He likely harbors romantic feelings for Hidaka, though the depth of their relationship remains ambiguous. As an adult, Miyao appears at age 28 in the spin-off series *Hi Score Girl Dash*.