Kasumi Nakasu is a first-year Nijigasaki High School student and the driving force behind its idol club. She joined with passionate dedication upon enrollment, becoming its most committed member. When the club dissolved, she persistently attended the empty club room alone, embodying its memory. Determined to revive it, she declared herself president as the sole member and recruited Ayumu Uehara and Yu Takasaki after overhearing their interest, maintaining her leadership as the club grew. Her personality blends a lively, kind nature with fierce competitiveness. She adores all things cute and idol-related, relentlessly striving to be number one, which sometimes leads to cheating or sabotage attempts. These schemes often fail or backfire, making her a sore loser. Despite this mischievous streak, she shows genuine kindness, like supporting fellow idols before performances or exchanging thoughtful gifts. She intensely dislikes the childhood bully-given nickname "Kasukasu" (meaning "just barely") and becomes visibly upset when called by it. Kasumi stands 155 cm tall with a slim build, grey-brown short hair, and light magenta eyes. Her signature accessory is a yellow star and crescent moon hairpin worn on her left side. Her image color is pastel yellow (#e7d600). Her hobbies include baking bread (2-3 hours) and watching idol videos (3-4 hours) on days off. She also jogs for one hour and dedicates a secret hour after 9:00 PM to undisclosed activities. Academically, she enjoys science and home economics but dislikes classical literature and history. She openly admires a specific senior, expressing a desire to do idol activities together. She collaborates with Dengeki Online alongside Karin Asaka and Setsuna Yuki, partly motivated by a rivalry with someone working with Famitsu. She founded and centers the subunit QU4RTZ. Her competitive drive manifests in attempts to undermine rivals, like sending scandalous information or placing bread rolls in their shoes. However, her plans frequently backfire or evolve positively, such as intended hate mail unintentionally becoming supportive fan mail. She occasionally proposes extreme ideas, like operating as a duo with her adored senior while excluding others, though these notions are typically dismissed.