Sister Princess
is Yui Horie's fourth solo album. Like sky, her third album, Rakuen also had a limited edition version with an alternate album cover and an exclusive photo book. The album includes the ending theme for the anime Jubei-chan 2, "Kokoro Harete Yo mo Akete" and the opening theme for the anime Sister Princess Re Pure Character's Part, "A Girl in Love" (used for DVD release only). Although the other sisters from Sister Princess Re Pure and even Ritsuko Okazaki, who wrote the song herself, have performed the latter song, the version on this album is the only full-length version ever recorded. The rest of the tracks are all original songs.