Kasumi Toyama is a third-year student at Hanasakigawa Girls' High School who passionately founded, leads, sings, and plays rhythm guitar for Poppin'Party, wielding a red ESP Random Star guitar. Her family consists of parents and a younger sister, Asuka, whom she affectionately nicknames "Aa-chan." Despite being older, their sibling dynamic often inverts expectations, with Asuka frequently displaying greater maturity.

A pivotal childhood moment shaped Kasumi: while stargazing, she heard a sparkling, heart-pounding sound she named the "Star Beat," igniting a lifelong quest to rediscover that sensation. This search led her to discover a star-shaped guitar in the pawnshop owned by Arisa Ichigaya's family and later to witness Glitter*Green's live performance. Believing music could recreate that kirakiradokidoki feeling, she formed her own band.

Kasumi sports brown hair styled with sections pulled up to resemble a star shape, secured by red star-shaped clips, and has purple eyes. Her evolving casual outfits include an early baby-blue T-shirt dress with a yellow flannel; a third-season ensemble of an orange hoodie over a white collared shirt, apricot skirt, and red-white sneakers; winter wear featuring a red shirt, yellow star-patterned sweater, and black skirt; and a summer polka-dot tee with a blue skirtall. She consistently accessorizes with star-themed necklaces.

Her personality radiates energetic optimism, curiosity, and an unyielding pursuit of excitement. She forms friendships effortlessly, maintaining a playful, affectionate demeanor that sometimes manifests as forgetfulness or unintentional teasing—particularly towards Arisa. She dislikes horror, fears flying, and struggles with swimming. Beneath her cheerfulness lies profound self-doubt during failures; harsh criticism after an audition once drove her to overpractice until she lost her voice, requiring bandmate reassurance to rediscover their shared joy.

Kasumi leads through action and intuition. She proactively recruited band members: convincing the reluctant Saaya to drum despite family obligations, recognizing Rimi Ushigome's bass potential, challenging Tae Hanazono to make her "heart race," and persistently befriending Arisa. Her decisiveness occasionally crosses into pushiness, like when insisting on normalcy during Arisa's withdrawal exacerbated tensions, though always driven by a desire to unite others—often catalyzing growth, as when Saaya realized she needn't shoulder burdens alone.

Her development involves navigating band conflicts, including Tae's temporary departure to Raise A Suilen, which culminated in Poppin'Party's disastrous cultural festival performance, teaching Kasumi the dangers of denial and the necessity of confronting problems directly. Later, she spearheads the band's participation in events like the Girls Band Challenge and Rockin' Star Festival, emphasizing collective experiences over outcomes. Her insight that live performances are "living creatures" requiring nurturing reveals unexpected depth. Academically, she becomes a part-time employee at the live house RiNG during her senior year and later attends university.

As Poppin'Party's primary lyricist, Kasumi translates everyday moments into songs by seeking the "sparkling" in ordinary life, drawing inspiration from memories and conversations, evident in lyrics like "Mae e Susume" and her first-year representative speech. Her songs often reflect band dynamics, symbolizing patience with Rimi's shyness or her stargazing promise with Saaya.

Relationships extend beyond her band. She shares a childhood friendship with Hagumi Kitazawa and connects with other band vocalists. Interactions with Asuka highlight a sibling yin-yang—Kasumi's exuberance contrasting her sister's stoicism—underpinned by mutual affection.

In an alternate storyline, Kasumi was initially portrayed as a bullied, reserved girl who hated singing and wore glasses, but this version was retconned in later media.