Yui Narusawa is an 18-year-old female with black hair styled in odango buns featuring side hair, spiky bangs, and wavy elements. She possesses red, tareme eyes. Standing 155cm tall, she has a slim physique, pale skin, and medium bust measurements of 83-56-84cm (B cup).

She addresses Ryunosuke as "oniisan" (big brother), having resided with him for years alongside her mother, Misako Narusawa. This arrangement places her in roles such as his classmate, daughter, or adopting an "imouto" (little sister) dynamic. Her personality traits include cheerfulness, being deredere (openly affectionate), friendliness, and naivety; she occasionally refers to herself in the third person.

She enjoys cooking and skiing. Her attire frequently includes a school uniform, miniskirt, sweater, turtleneck, tank top, apron, bow tie, loafers, hair ribbon, yukata, one-piece swimsuit, coat, and towel. She is also seen carrying a bag and often becomes a subject of teasing during social interactions.

Within the Dōkyūsei media, she serves as the main character in "Doukyuusei 2," "Doukyuusei 2 Special Disc," and "Elf All Stars Datsui Jan." Her role in "Dōkyūsei: Climax" is a cameo appearance.