Kokoro Tsurumaki, a third-year student at Hanasakigawa Girls' High School, leads the band Hello, Happy World! as its charismatic vocalist. Hailing from immense wealth, her family’s holdings span a private beach, mountain, and a vacation retreat on "Happy Happy Island." Privileges like a personalized train pass emblazoned with "Tsurumaki" and membership in the school’s faculty-established Astronomy Club—rumored to placate her family—underscore her influential lineage.

Driven by relentless optimism, Kokoro dedicates herself to spreading global joy through music. Her insatiable curiosity propels hyperfixations, from zombie fascination sparked by horror films to mastering skills like horseback riding or taiko drumming with startling speed. Though a natural leader, her scatterbrained tendencies require bandmates to ground her grand visions in practicality. She asserts an upbringing free from sorrow or stress, unaware of such concepts until others explained them.

Waist-length platinum blonde hair frames large yellow eyes, paired with uneven bangs that defy symmetry. Her casual ensemble—a red-and-white striped shirt under denim overalls, accented by red ribbons—gives way to seasonal flair: snowflake-knit sweaters in winter or nautical-themed summer wear.

Surprising agility lets her perform acrobatics like backflips even in traditional attire, while rumors speculate her rapid skill acquisition hints at hidden genius. She disregards formalities, addressing all by first names with cheerful familiarity.

In *BanG Dream! Pretty Cure!* spin-offs, she embodies Cure Popcorn, channeling explosive, joy-themed powers that mirror her exuberance. Narratively, she sparks growth in peers like Misaki and Kanon through unwavering positivity, remaining a static beacon of cheer amid challenges—catalyzing change in others while her own arc resists introspection.