Yui Narusawa stands 155 cm tall with a slim physique, pale skin, and measurements of 83-56-84 cm (B-cup bust). She possesses long black hair typically styled in odango buns, featuring spiky bangs and wavy side hair. Her eyes are red and exhibit a tareme (drooping) shape.

She frequently appears in a school uniform, often accessorized with hair ribbons and bow ties. Her documented attire also includes aprons, coats, loafers, miniskirts, turtleneck sweaters, sweaters, tank tops, yukata, one-piece swimsuits, and towels. She is sometimes shown carrying a bag.

Her personality is cheerful, friendly, naive, and deredere (affectionate). She habitually refers to herself in the third person. Her activities include cooking and skiing, and she is noted as subject to teasing by others.

Yui resides with Ryunosuke (the protagonist) and her mother, Misako Narusawa. She addresses Ryunosuke as "big brother" (お兄さん, oniisan), though their biological relationship is unspecified; this dynamic incorporates an element of "imouto act" (little sister role-play). Socially, she is Ryunosuke's classmate and a twelfth grader.

Narratively, she serves as a central romantic interest for the protagonist during his winter break. The storyline involves developing relationships leading to graduation decisions, with Yui being one of fifteen potential romantic partners. Her character appears across original video animation (OVA) adaptations.

Collectible materials feature her on original animation cels from OVA productions, depicting various scenes and outfits consistent with her design.