Kakyuu is the princess of Kinmoku, a planet outside our solar system. She's best known for smelling like fragrant olive blossoms, which show up constantly in her design and character themes. The whole thing's actually a clever pun—her planet's name comes from kinmokusei, the Japanese name for these flowers, with "sei" meaning planet or star added on. When she introduces herself, she mentions her kingdom as Tankei, which is another word for the same flower.
Kinmoku is also where the Sailor Starlights come from. They're her protectors and basically spend the entire story looking for her. How their story actually plays out differs pretty significantly between the manga and anime versions.
Content compiled by AnimeList.moe from publicly available sources.
