Kasumi Nakasu, a first-year Nijigasaki High student, bursts onto the school idol club scene as its most fervent advocate. When the group faces disbandment, she appoints herself president, spearheading its revival by recruiting members like Ayumu Uehara and Yu Takasaki. Fueled by a lifelong idol aspiration, her resolve wavers briefly when the club falters, exposing vulnerabilities beneath her bold exterior.

Driven by a fiercely competitive streak, Kasumi relentlessly pursues top idol status, devising playful schemes—from distracting rivals with treats to crafting faux-critical letters that unintentionally uplift recipients. Beneath her mischievous tactics lies a supportive core: she bolsters anxious teammates pre-performance and exchanges meaningful tokens with peers. The nickname "Kasukasu," tied to childhood taunts implying insufficiency, provokes sharp reactions, underscoring lingering sensitivities.

Her passions include baking artisanal bread, analyzing idol performances, and clandestine nightly endeavors she shrouds in mystery. Academically, she thrives in science and home economics yet stumbles over classical subjects, mirroring her future-oriented outlook. Physically, she cuts a distinctive figure with cropped grey-brown hair, light magenta eyes, a star-shaped hairpin, and a petite 155 cm frame.

Initially clinging to the club’s remnants alone, Kasumi gradually embraces collaboration, valuing collective success over solo glory. She openly admires peers like Karin Asaka’s sophistication and Setsuna Yuki’s stage mastery, masking envy with playful banter. Her dynamic with the club’s protagonist oscillates between teasingly dubbing them "Senpai" and proposing exclusive partnerships, though she ultimately champions group unity.

Kasumi’s journey navigates the tension between individual ambition and communal bonds. Though her ploys frequently misfire, they expose a sincere drive to shine while uplifting others, cementing her role as the club’s spirited anchor.