Kokoro Tsurumaki, a third-year student at Hanasakigawa Girls' High School, leads the band Hello, Happy World as its vocalist. Her lifelong ambition centers on making the entire world smile. Born into immense wealth, she resides in one of the area's largest houses and owns additional properties including a private beach, a mountain, and a vacation home on "Happy Happy Island." Her personality radiates idealism, charisma, and relentless optimism, effortlessly drawing others into her energetic pace. A naturally imposing demeanor allows her to pursue desires with minimal resistance. Intense curiosity leads her to embrace diverse experiences, such as developing a fondness for zombies after encountering a horror series. Despite leadership qualities, she relies heavily on bandmates for focus due to her scatterbrained nature. Kokoro claims she has never felt sadness and only learned of stress from Marina Tsukishima. Her affluent upbringing leaves her unaware of everyday norms for less wealthy individuals. Within Hello, Happy World, she drives the band’s mission to spread joy through music and whimsical projects like surprise public performances or elaborate smile-distributing events. Bandmates, particularly Misaki Okusawa, ground her grand visions. Kokoro exhibits extreme open-mindedness, enthusiastically trying almost anything—from instantly mastering skills like horse riding, taiko drumming, or juggling to exploring unfamiliar entertainment genres. She addresses everyone informally, using first names without honorifics even for adults like Marina. Kokoro shares a unique rapport with Poppin'Party’s Kasumi Toyama due to their similarly hyperactive and eccentric traits, often exchanging each other’s band catchphrases. She is the sole member of Hanasakigawa’s Astronomy Club, established by faculty to accommodate her late-night school activities and avoid conflicts with her influential family. Distinctive traits include a personalized train pass displaying "Tsurumaki" instead of a balance, exceptional physical agility enabling acrobatics like backflips in restrictive clothing, fan theories suggesting latent genius-level aptitude beneath her oblivious demeanor, and a frequent habit of borrowing picture books from libraries.