Yui Narusawa is a young woman living with Ryunosuke and her mother, Misako Narusawa. She consistently addresses Ryunosuke as "big brother" (お兄さん, oniisan), reflecting their long-standing cohabitation and familial bond. Standing 155 cm tall with a slim build and pale skin, she possesses medium breasts (B cup, measurements 83-56-84 cm). Her look includes long black wavy hair styled in odango buns, spiky bangs, and side hair, complemented by red tareme eyes.

Her personality is cheerful, friendly, and naive, often exhibiting a deredere, affectionate demeanor. She habitually refers to herself in the third person. Yui enjoys activities like cooking and skiing. Within the household, she holds the role of a daughter and performs an "imouto act," a sister-like role, toward Ryunosuke.

In the OVA *Dōkyūsei 2 Special: Sotsugyōsei*, Yui harbors romantic feelings for Ryunosuke, which remain unreciprocated throughout most of the story, depicting her emotional struggle with this unrequited affection. While her living situation with Ryunosuke is noted, the specific circumstances or duration of their cohabitation are not detailed. Her romantic arc resolves when Ryunosuke unexpectedly kisses her in the final episode.