Mio Mizukoshi is a supporting character in the series, a third-grade elementary school student and a close friend of Ai Hinatsuru. She first meets Ai during a shogi match at the Kansai Shogi Hall, and after Ai becomes the live-in disciple of Yaichi Kuzuryu and moves to Osaka, Mio and Ai become classmates at the same elementary school. Mio has short brown hair held in place with two black X-shaped hair clips, and brown eyes. She typically wears a red jacket and brown shorts, matching her active and sporty demeanor. She is described as lively, energetic, and athletic, with a natural flair for leadership that makes her the de facto leader of the JS Study Group, a small club of elementary school girls who study shogi together. Her speech is occasionally marked by a slight mispronunciation, such as saying "mijukoshi" instead of "Mizukoshi" and "kujuryu" instead of "Kuzuryu". These traits, combined with her tomboyish appearance, contribute to her approachable and cheerful personality.

In terms of personality, Mio is optimistic, outgoing, and always ready to energize those around her. She enjoys physical activities and once rode a bicycle around Lake Biwa during summer break. She is also caring and loyal toward her friends, especially Ai, from whom she draws inspiration even in difficult moments. Initially, Mio struggles with feelings of inferiority when she repeatedly loses to Ai's overwhelming natural talent in shogi, leading to moments of self-doubt and lost confidence. However, this changes when both Ai and Yashajin Tengai achieve professional female shogi player status. Inspired by their success, Mio decides to seriously aim for a professional career in shogi herself, marking a key turning point in her character development.

Her motivation shifts from simply enjoying shogi as a hobby to actively pursuing the goal of becoming a professional female shogi player. This ambition is driven partly by her friendship with Ai and the desire to keep up with her, as well as a genuine love for the game. Mio is an all-rounder shogi player, capable of using both static rook and ranging rook strategies, which gives her a versatile playing style. She does not have supernatural abilities or a unique gimmick; her strength lies in her adaptability and determination to improve through hard work. Over the course of the story, she matures from a carefree sporty girl into a more focused and resilient player, learning to overcome her insecurities and support her friends both on and off the shogi board. Her role in the narrative is primarily that of a supportive friend and a relatable foil to the prodigious talents of Ai and Tengai, illustrating the challenges and rewards of pursuing excellence through steady effort.