Mio is Yusuke Yagami's cousin, sharing an ongoing sexual relationship with him. Yusuke engages in these physical encounters primarily for gratification, distinct from his wider misogynistic views. This relationship provides one of Yusuke's few consistent outlets, though it remains devoid of deeper emotional connection and is fundamentally carnal.

Later, Mio directly challenges Yusuke concerning his manipulation and abuse of his stepfamily. This confrontation sparks a moment of self-reflection in Yusuke, contributing to his eventual recognition of his own destructive nature. Her secondary role acts as a catalyst for Yusuke's fleeting guilt before his downward spiral intensifies.

Mio's background beyond her familial connection to Yusuke and their physical relationship is not elaborated. Her character functions primarily to underscore Yusuke's psychological complexities and the cyclical nature of his trauma, rather than as a fully developed individual with independent motivations or history.