Kokoro Tsurumaki

Birthday - August 8, Height - 155 cm, Blood Type - B, Class - 1st year C class (Season 1) → 2nd year B class (Season 2) → 3rd year E class (Season 3), Nickname - Smiling Wave Attack

Leader and vocalist of Hello, Happy World! A student in grades 1→2→3 at Hanasakigawa Girls' High School.

She is a young lady from a distinguished family, known to have celebrities attend her birthday parties and acquaintances with kings of European kingdoms, and is always accompanied by bodyguards referred to as "men in black suits." Due to her status, a teacher who feared her created an Astronomy Club for her, where she is the sole member. She was in the same class as Misaki in the first year, and later with Rimi and Saaya. Known as "Hanasakigawa's Different Dimension" due to her high-spirited nature and bizarre behavior, she quickly became friends with Kasumi and Hagumi.

She speaks informally to seniors and superiors without using honorifics, but characters who are particularly conscious of maintaining discipline, like Sayo, do not reprimand her, suggesting that the reasons might be her feminine speech or her unconventional thinking rather than just her noble status.

Her physical and athletic abilities are exceptionally high; she can jump from the fourth floor of a building and land unharmed, or perform a backflip off the school fence. She routinely performs acrobatic moves that only top gymnasts can execute.

She has a keen observational ability, instantly detecting if someone has something to hide. However, she can be forgetful, such as planning a live performance at a high place forgetting Kaoru's fear of heights.

She is not picky about food and eats well. Her hobby is looking for fun activities.

She is unaware of Michelle's true identity. Although Misaki has revealed it to her, she stubbornly refuses to believe it. The aforementioned men in black also inexplicably cooperate fully in hiding Michelle's identity. Initially, she couldn't even remember Misaki's name or face.