The term "Kaoru Imouto" originated from a Japanese visual novel called "Saya no Uta" (The Song of Saya), which was released in 2001. The story revolves around a high school student named Fuyou Shiki who becomes involved with a mysterious girl named Kaoru Minami, who is also his classmate. As the story progresses, Kaoru becomes a central character, and her relationship with Shiki is explored in depth.