Mio Kusakai is a student at the Setouchi Keijo Training School, where she is ranked first in the Elite Class. Of British descent through her mother, she has blonde hair, blue eyes, and is fluent in English. Before entering the world of keijo, Mio was a former Olympic gold medalist in backstroke swimming, a background that gives her an exceptional sense of balance and a sharp analytical mind. These athletic foundations translate seamlessly into her keijo career. She is an infighter, meaning she specializes in close-quarters combat, and possesses a wide range of special techniques. Mio's personality is that of a composed and highly confident elite; she carries herself with the poise of someone accustomed to being at the top. Her primary motivation is to succeed as a professional keijo player, driven by the same competitive fire that made her a champion swimmer. In the story, she serves as a supporting character and a significant rival to the protagonist, Nozomi Kaminashi. Their relationship is marked by mutual respect and intense competition, pushing each other to improve. Mio initially enters the professional ranks as a B-Rank player after graduating from Setouchi and later advances to A-Rank, indicating steady growth and development in her abilities as she continues to refine her skills and strategies.