Mio is a beautiful young girl of average height, with long waist-length red hair and violet eyes. She is noted for having a distinctly voluptuous body for someone her age. Mio has been seen wearing a number of outfits throughout the series, with several casual attires at home that are usually form-fitting or revealing, as well as a uniform for attending school. After forming a contract with Basara, Mio gains a choker-like tattoo around her throat which takes the appearance of a love heart with two prongs extending around either side of her neck.
Mio normally displays a calm demeanor, preferring to remain quiet when around others; however, she is prone to outbursts when faced with inappropriate situations such as finding erotic magazines underneath Basara's bed or being left at the mercy of Maria's succubus curse. Despite this, she is also caring and affectionate, genuinely appreciate Basara's kind treatment and protection as well as feeling uncomfortable about having to deceive innocent people. Deep down she is afraid of her current situation and constantly being under threat, but when faced with danger she won't hesitate to face the challenge.