Koji is a member of the Ganma Army and the older brother of Umako Harada. He had an unusual start in life: at the age of three he fell into a pond and was swallowed by a koi fish. The fish was later caught and sold to a wealthy couple, who adopted and raised him. As a teenager, he played baseball in high school, and his skills on the field led to him being scouted and recruited by the Ganma Army. This background in athletics carries over into his combat style, allowing him to use his throwing and striking abilities in military situations. A defining aspect of his personality is his relationship with his sister Umako, and his deep attachment to her is a source of melancholy that heavily influences his behavior. He is an adult man with short blue hair and gray eyes.