Kakyuu is the princess of Kinmoku, a fictional planet outside of the solar system. She is said to smell like the flowers of the Fragrant Olives, which are a recurring motif in her character design. The name of her planet is a pun on the name of these fragrant blossoms, which are called kinmokusei in Japanese--the suffix sei, when added to Kinmoku, means "planet" or "star." When Kakyuu first introduces herself, she calls her specific kingdom Tankei, a homonym of another word which also means the Fragrant Olives shrub.
Kinmoku is also the home of the Sailor Starlights, who are Kakyuu's protectors and spend much of the story searching for her. The details of their collective story vary between the manga and the anime.
At some point before the story begins, Kinmoku is attacked and destroyed by Sailor Galaxia, leaving almost no survivors. Kakyuu herself travels to Earth and hides within an incense burner protected by a small child, ChibiChibi. Having lost contact with her, the Sailor Starlights follow behind, searching desperately.
In the manga Kakyuu is injured in the battle with Galaxia, and cannot reveal herself until her wounds are healed. She travels to Earth because she senses the birth of the Silver Moon Crystal (which is an evolution of the original Silver Crystal) there. She turns out to be Sailor Kakyuu, and reveals to Sailor Moon that her own lover had died in the war against Galaxia. She eventually accompanies Sailor Moon to Zero Star Sagittarius to confront Galaxia, but is brutally killed by Sailor Chi. She dies in Sailor Moon's arms, saying that she want to be reborn, maybe in a world without war, but at the very least to be with everyone again.
In the anime, no mention of battle-wounds is made, and her purpose in going to Earth is to find the "Light of Hope" and hide from Galaxia. During her time under Chibichibi's care, she is aware of the Sailor Starlights searching for her, but cannot reveal herself too soon. She finally emerges to save Sailor Moon and the others from a black hole and resumes leadership of the Starlights. Not long after Kakyuu is found, Galaxia appears in battle and personally takes Princess Kakyuu's star seed. (Unlike the manga, Kakyuu never transforms into a Senshi form, although she is shown to have a "true star seed.") Kakyuu sets up a forcefield that is able to block Galaxia's attacks for a short time, but it does not save her. Before dying, Kakyuu sees in a vision that ChibiChibi herself is the Light of Hope. After Sailor Moon defeats Chaos, Princess Kakyuu is revived. She and her Starlights return to their home to rebuild and start over.
