Kasumi Toyama is a third-year student at Hanasakigawa Girls' High School and the leader, vocalist, and rhythm guitarist for the band Poppin'Party, for which she also writes the majority of the lyrics. She lives with her parents and a younger sister, Asuka; their relationship often blurs the line as to which sister is truly the more responsible one. As a child, while gazing at the stars, Kasumi heard a sparkling sound she calls the "Star Beat," sparking a lifelong pursuit of that feeling—a sensation she believes she rediscovered upon first seeing her star-shaped guitar and forming Poppin'Party. She is childhood friends with Hagumi Kitazawa, though Hagumi only recognized her after Kasumi changed her hairstyle from the twin tails she wore in her youth. Kasumi has brown hair worn loose with sections pulled up to form a star shape, held in place by red star-shaped clips, and purple eyes. Her typical casual outfit is a baby blue T-shirt dress with a yellow flannel shirt tied around her waist, a silver star necklace, and brown boots. Another frequent combination includes a white collared shirt under an orange hoodie with a yellow hood, an apricot miniskirt, and red-and-white sneakers. In colder weather, she wears a red collared shirt under a yellow sweater adorned with white stars, a black miniskirt, and tights. One summer outfit consists of a white polka-dot T-shirt with transparent sleeves and a blue skort, accessorized with a three-star necklace. Her personality is intensely cheerful, high-spirited, playful, active, and optimistic, with a knack for finding excitement and passion in nearly everything. She makes friends effortlessly and gets along with all personality types, sharing a particularly energetic wavelength with Kokoro Tsurumaki and Tomoe Udagawa. Kasumi can be forgetful, frequently attending to important matters at the last minute unintentionally, and has a tendency to believe unusual or far-fetched explanations. She often teases Arisa Ichigaya, though it is unclear whether this is intentional or born of obliviousness, and can be melodramatic on occasion. A determined and persistent individual, Kasumi’s powerful desire to unite her friends and steer them toward a shared musical goal sometimes comes across as selfish or forceful, such as when she insisted that Saaya Yamabuki become the band’s drummer despite Saaya's family obligations. This same drive, however, inspires those around her, as when she helped Rimi Ushigome overcome deep-seated confidence issues. An alternate storyline from the original manga depicts Kasumi with a completely opposite personality: depressed, lonely, and reserved, with very low self-esteem and little presence, a result of childhood bullying. This version of Kasumi hated singing, viewed herself as below average, and considered her hobby of collecting star-shaped items childish. She wore glasses throughout her life until switching to contact lenses in high school in an effort to reinvent herself. Kasumi works part-time at the live house RiNG alongside Saaya Yamabuki and Taki Shiina. Her hobbies include karaoke, going on adventures, and trying new experiences. She enjoys eating French fries and white rice but dislikes natto. Unlike her sister, who was a member of the school swimming club, Kasumi is not a strong swimmer. She dislikes all forms of horror and becomes frightened when flying on airplanes. She possesses a unique ability to detect, by smell alone, when Saaya’s family bakery has produced a new type of bread. Her signature instrument is a red ESP Random Star guitar, which she first discovered in a pawnshop owned by Arisa’s family. The formation of Poppin'Party was inspired after she saw a performance by the band Glitter Green at the live house SPACE. Despite her enthusiasm, Kasumi initially lacked technical skill, unable even to tune a guitar properly, and faced significant setbacks—including losing her voice from stress after being told she was the weakest member during Poppin'Party’s first audition at SPACE. Supported by her friends, she overcame this challenge, leading to the band’s eventual success and a triumphant performance at SPACE. As leader, she spearheads many band activities and cross-group collaborations, such as helping organize the Girls Band Party event at the live house CiRCLE. During her senior year of high school, she becomes a full employee at RiNG. In the chibi-style mini-anime series "BanG Dream! Girls Band Party!☆PICO ~Ohmori~," Kasumi appears alongside other band members in comedic scenarios, often set at CiRCLE. In one episode, she leads an initiative to create a world-class bread for Poppin'Party called "Poppin'Pastry."