Kokoro Tsurumaki is a student at Hanasakigawa Girls' High School and the energetic leader and vocalist of the band Hello, Happy World!. She is the only daughter of an extremely wealthy family, a status that allows her to pursue any idea that catches her interest without practical limitations. Her lifelong goal is to make the entire world smile, and this mission defines both her personal actions and the direction of her band. She is a free-spirited and naturally entertaining person who is not bound by convention, approaching life with boundless curiosity and an unwavering positive attitude. Her hobby is searching for fun things to do, and whenever something sparks her desire to act, she will go to any lengths to realize it. This relentless enthusiasm makes her highly sociable and quick to form friendships, though her unconventional thinking often leads her to recruit others into her spontaneous, large-scale schemes. Within the story, Kokoro serves as the driving force behind Hello, Happy World!, continually pushing the band to expand its activities beyond typical music performances in order to spread happiness across the globe. Key relationships include her bandmates: guitarist Kaoru Seta, bassist Hagumi Kitazawa, drummer Kanon Matsubara, and DJ Misaki Okusawa, with whom she shares a classroom alongside Hagumi. Her childhood friend is a character named Charlotte. Throughout her development, Kokoro learns to balance her single-minded pursuit of fun with a deeper understanding of teamwork and the emotions of those around her, growing into a more empathetic leader while retaining her core optimism. Notable abilities include her charismatic stage presence and her powerful, emotive singing voice, which she uses to uplift audiences and embody the joy she aims to create.