Mio Sugiyama is a high school student and an active member of the school tennis club. She is the younger half-sister of the protagonist, Shuuichi, with whom she shares a sexual relationship. Mio often initiates these encounters with an aggressive approach, particularly after tennis practice in the club's changing room. She has also had sexual experiences with another character, Ai Saeki, both with and without Shuuichi's involvement.

Physically, Mio stands 150 cm tall and weighs 42 kg, with a slim build and a pale complexion. Her body measurements are 74-58-74 cm. She has straight brown hair styled with blunt bangs and long twin tails that reach her waist. Her eyes are brown and upturned.

Mio exhibits a proactive and assertive personality in her interactions, particularly within her romantic and sexual relationships. She is noted as being spoiled. Her typical attire includes her school uniform, a tennis outfit, a miniskirt, thigh-high stockings, Gothic style dresses, a kemonomimi headband, and various cosplay outfits.