Kōji Mihama is a member of the successful prism show unit Over the Rainbow. He was born on January 14 with O blood type and is right-handed. His father, Takeyuki Mihama, taught him cooking, baking, and guitar, fostering a bright childhood. Takeyuki's death had a profound effect on Kōji, shifting his personality to become quiet and gloomy, and he rarely smiled. In the events leading up to King of Prism: PRIDE the HERO, Kōji had a falling-out with fellow Over the Rainbow member Hiro Hayami over the song "pride," but that conflict is resolved, and Kōji regains his original lively personality.

Kōji is a genius songwriter and composer. The songs he writes possess a charm that is sometimes described as devilish, and he receives a flood of offers from overseas, keeping him constantly busy. In the film, when rival school Schwarz Rose's schemes threaten Edel Rose with a massive debt, Over the Rainbow goes on an indefinite hiatus to protect the school. Kōji takes on the responsibility of repaying part of the debt by traveling to Hollywood to produce film music, while fellow members Hiro and Kazuki pursue their own paths. His motivation is driven by loyalty to his friends and the school, and by the desire to reconcile with Hiro.

Kōji is tall, with chartreuse eyes and shoulder-length dim gray hair that he ties into a ponytail. He wears a dog tag that belonged to his father and a purple Over the Rainbow wristband that matches those of Hiro and Kazuki. He is in a relationship with fellow musician and prism star Ito Suzuno. Throughout the story, Kōji's development centers on reconnecting with his original self, resolving past grievances, and using his extraordinary musical talent to help those he cares about. His abilities extend beyond songwriting to include cooking, which he learned from his father, and he eventually hosts a TV cooking show. By the end of the film, Kōji completes all his overseas composing orders, reaffirming his identity as a genius artist, and reunites with Over the Rainbow.