Hatsune is Kazuki's big sister, who is constantly trying to get Kazuki and Yuuji together and inadvertently puts them both into embarrassing and lascivious situations. There are some indications that she considers the mishaps (and the arguments following them) funny. She is extremely popular at school with many clubs wanting her to become a member. Hatsune is good at everything she does, from cooking to sports, and is extremely well-endowed. She likes to tease her sister by showing her how infatuated Yuuji is with her breasts. Hatsune frequently engages in cosplay around the house and can, from time to time, be quite frightening to the people around her. Unlike her younger sister and Marie, Hatsune is perfectly unafraid of insects - or anything else. Although it's implied that Hatsune's true weakness is worrying about her sister's health. When Kazuki fell ill with a cold, Hatsune became comically uncoordinated and unskillful in cooking. She occasionally (and sarcastically) appears to have an attraction to her younger sister.

- Wikipedia