Tsurumaki Kokoro is a happy-go-lucky second-year student at Hanasakigawa Girls' High School and the leader and vocalist of Hello, Happy World!

An idealistic and charismatic young girl coming from a ridiculously rich family, Kokoro dreams of making the entire world smile from music. She effortlessly attracts others to her due to her bright and bubbly nature, and often manages to get them swept up into her easygoing pace. Because of her charismatic and naturally imposing personality, she is often able to do and get what she wants. She is also extremely curious and easily amazed, charmed by new things she discovers in the world around her.

Although Kokoro has the optimism and charm of a natural leader, she is also quite scatterbrained and ditzy, and would not be able to get much done without the others in the group helping her. She also has said that she doesn't know what it feels like to be sad, and does always her best to combat any kind of negative feelings.