Kasumi Nakasu is a first-year Nijigasaki High School student and central figure in its School Idol Club, with pastel yellow as her image color. She founded and leads the subunit QU4RTZ. Originally joining to pursue idol dreams, she became the club's sole member after its disbandment, self-appointing as president. Kasumi later revived the club by recruiting Ayumu Uehara and Yu Takasaki, retaining leadership as membership grew.

Her personality blends vibrant enthusiasm with fierce competitiveness. She adores cute and idol-related things, often showing kindness by encouraging nervous teammates before performances. Yet she despises losing, sometimes employing scheming or sabotage to win—though her plans typically fail without genuine malice. She intensely dislikes the nickname "Kasukasu" due to past bullying.

On days off, she spends 2-3 hours baking bread and 3-4 hours watching idol videos. She jogs for one hour daily and reserves a secret hour after 9:00 PM for undisclosed activities. Her preferred school subjects are science and home economics, while she avoids classical literature and history.

Physically, Kasumi stands at 1.55 meters with grey-brown hair, light magenta eyes, and a slim build. She sports a distinctive yellow star-and-crescent hairpin on her left side.

Beyond school, she collaborates with Dengeki Online alongside Karin Asaka and Setsuna Yuki, admiring Karin's maturity and Setsuna's talents. In the *Nijiyon Animation 2* spin-off, she partners with Karin to strategize cuteness enhancements, reflecting her idol self-improvement drive.

Her background reveals a pivotal motivation: idolizing a specific senior who inspired her initial hesitation to pursue school idol activities. This admiration fueled her commitment to reviving the club and her aspiration to become a top idol.