Kasumi Toyama leads the band Poppin'Party as vocalist, rhythm guitarist, and lyricist. She attends Hanasakigawa Girls' High School as a third-year and works part-time at the live house RiNG with bandmate Saaya Yamabuki and staff member Taki Shina. Her family consists of her parents and younger sister Asuka, whom she nicknames "Aa-chan"; their sibling dynamic often obscures clear lines of responsibility. A childhood moment under the stars, where she sensed a "sparkling heartbeat" or "Star Beat," sparked her ongoing quest to recapture that feeling. She linked it to music upon discovering a star-shaped guitar at Ryuuseidou pawn shop and founding Poppin'Party.

Her appearance includes brown hair styled with sections pulled up by red star-shaped clips, creating a silhouette sometimes mistaken for cat ears. She has violet eyes and expressive features. Her outfits evolve across media: early casual wear features a baby-blue T-shirt dress with a yellow flannel, while later designs include an orange star-motif hoodie, checkered skirts, and seasonal variations like polka-dot summer dresses or star-adorned winter sweaters. She consistently accessorizes with star-themed jewelry, particularly a silver necklace.

Kasumi radiates cheerfulness, optimism, and relentless enthusiasm. She forges friendships effortlessly, connecting with diverse personalities across bands like Yukina Minato and Maruyama Aya (whom she calls senpai). Her sociability extends to her bandmates: she persistently recruited Arisa Ichigaya, shares energetic camaraderie with Kokoro Tsurumaki and Tomoe Udagawa, and often shows physical affection like hugging Rimi Ushigome or Saaya. However, her impulsiveness and occasional obliviousness strain relationships, such as when her pushiness during the "Double Rainbow" event hurt Arisa, or when her denial of Tae Hanazono's conflicts led to a disastrous cultural festival. These flaws manifest in broader struggles, including stage fright, vocal paralysis during Poppin'Party's SPACE audition due to fear of inadequacy, and neglecting studies for band priorities.

Her development involves confronting these challenges. After the SPACE failure, she isolated herself, practicing guitar until her fingers bled and losing her voice. This crisis resolved only when Poppin'Party shared their feelings, leading to a rewritten, collaboratively lyrical version of "Mae e Susume." Season 3 highlights her matured leadership: while Arisa strategized for the Girls Band Challenge, Kasumi encouraged risk-taking by proposing performances at new venues like CiRCLE and DUB, framing growth through shared experiences. Her lyricist process translates ordinary moments—bakery visits, school routines—into "sparkling, heart-pounding" emotions for songs like "Girl’s Code" and "Kizuna Music," a method her teacher Nakamura praised as genius.

Key relationships define her arcs. With Arisa, she balances teasing with deep mutual reliance; their bond strengthens through conflicts, like Kasumi's tearful hug after getting lost. She frequently embraces and encourages Rimi, especially regarding her Kansai dialect. Sister Asuka anchors poignant memories: Kasumi kissed her cheek during her voice-loss crisis, relies on her for comfort (like resting on her lap while reminiscing their shared stargazing), and playfully pesters her to join school clubs. Band chemistry emerges through collaboration, like guitar lessons with Tae or reconciling with Saaya after clashing over family expectations.

Trivia notes: She dislikes horror and air travel, swims poorly compared to Asuka, and possesses an idiosyncratic ability to detect new bread varieties at Saaya’s bakery by scent. Her signature "Random Star" guitar received a real-life replica by ESP Guitars. Taiwanese and Chinese fandoms humorously describe her hairstyle as resembling "sticky rice dumplings."