Kakyuu is the princess of Kinmoku and who the Starlights guard. In the anime, she hides in incense burner for quite awhile she apolgizes to Seiya she heard their song but if Galaxia had found her it would've been the end of everything. Kakyuu's mission while on Earth is to search for the Light Of Hope...before leaving the burner who was protected by Chibi Chibi she healed herself.
Kakyuu wished the same as Usagi that they could work together to find the Light Of Hope and end the war. Her Starlights and Inners in 194, but Healer says "We don't need it!" Due to the fact Kakyuu is well and they can easily recover their planet but without the Light Of Hope that isn't possible. Sadly, the Starlights and inners couldn't work together till aftyer 196...
In the manga, Kakyuu came to Earth to find Sailor Moon the lengendary sailor solider to help end the wars and save the galaxy. Like in the anime, she hid in the insence burner that was held onto by Chibi-Chibi. But Chibi-Chibi actually found Kakyuu in a playground in the manga and gained the burner to keep her safe.
Seiya also made a connection bewtween the burner and the scent of their princess, the three try to get the burner from Chibi Chibi (however in the anime it's only Taiki and Yaten) but Kakyuu comes out of the burner explaining her wounds took long to heal then she thought they would.
Kakyuu explains what Shadow Galaxia actually is to Usagi after a battle with Sailor Tin Nyanko, she also tells Usagi that her final target is her. Usagi now decidedes to fight and Kakyuu leads her to Sagittarius Alpha Star. In the manga more is revealed about the princess, Seiya tells Usagi during the battle on Kimoku Kakyuu lost her lover.
Kakyuu's name roughly translates to Fireball Princess.
