Princess Kakyuu, sometimes referred to as "Princess Fireball," is the ruler that the Sailor Starlights are sworn to protect. Her mission while on Earth is to find the so-called "Light of Hope" - the light that is capable of defeating Galaxia. The anime doesn't make it very explicit whether Kakyuu knows what the Light of Hope is supposed to be, how it's going to manifest itself, or even how to go about looking for it. In fact, whether she actually succeeds in her mission or not is still up for debate.
During most of the season, Kakyuu is in hiding and the Sailor Starlights, her guardians, are searching for her with their music. The Three Lights hope to contact Princess Kakyuu with their songs. Eventually, Chibi-Chibi discovers an antique incense burner, and takes quite a shine to it. She begins carrying the incense burner around, and then some very strange things start to happen. Etheral butterflies flit around Usagi and her friends, and the Three Lights notice the scent of fragrant olive blossoms surrounding Chibi-Chibi. Finally, Taiki and Yaten recognize the incense burner as belonging to Princess Kakyuu.
"There has been a holy war going on for a long time throughout the galaxy, the Sailor Wars. The Sailor Senshi have always fought against evil. The legendary strongest Sailor Senshi sealed away Chaos, which was the source of evil in the galaxy. But now, Chaos reappears in the galaxy. Galaxia wants to collect all the starseeds and plunge the galaxy into Chaos. We still have hope, as long as we have the Light of Hope that the legendary Sailor Senshi left. I came to this planet secretly to find that power."
Kakyuu eventually reveals herself to the Starlights. She was hiding inside the incense burner while she watched over the Starlights and Sailor Moon.
Kakyuu tells the Senshi of Earth about the Sailor Wars, and enlists their aid in the search for the Light of Hope. Recognizing that time is short, the Three Lights plan one last concert during which they plan to attract the Light of Hope with the power of their music.
Thus, the Sailor Senshi, Kakyuu, and Chibi-Chibi gather to attend the final concert of the Three Lights. The music plays, the crowds cheer, and Chibi-Chibi...ah, starts glowing. But nobody really notices, because only a few moments after this phenomenon begins, explosions rock the stadium. Sailor Tin Nyanko makes her entrance, hissing and howling and out for blood.
The sky turns dark. Thunder rumbles, and lightning flashes. The concert, needless to say, is over. The greatest fears of the Sailor Senshi have been realized: Galaxia is descending to the Earth.
The Golden Senshi appears before Sailor Moon and the Senshi. She makes an unforgettable first impression by promptly killing Tin Nyanko. Next, Galaxia singles out Princess Kakyuu. "Princess of the fragrant olives," Galaxia says as way of a formal greeting. Galaxia strikes, and Kakyuu stands her ground. The Starlights try to protect her, but she commands them to run away. After a brief struggle, Galaxia overwhelms Kakyuu and takes her starseed. Kakyuu collapses, fading into oblivion.
Is this the end of the Princess of Kinmoku?
As she dies, Kakyuu sees of vision of Chibi-Chibi floating toward her. "A warm light," she whispers with her dying breath.
Chibi-Chibi reaches a hand toward the Princess, and murmurs, "It's all right..."
Kakyuu utters her last wish for the Starlights, and then dies in their arms.
Chibi-Chibi was the Light of Hope all along. She was the manifestation of the starseed that the legendary Sailor Galaxia left behind to defeat Chaos. Well, it's a bit more complicated than that, but... I can't spoil the ENTIRE series on this page, now can I?
In the end, Sailor Moon does manage to defeat Chaos. All of the starseeds that Galaxia collected are restored, including Princess Kakyuu's. The Princess appears again before the Starlights. To say that they are happy to see her is a bit of an understatement. "It's a miracle," whispers Starhealer as tears slide down her cheeks.
Kakyuu and the Starlights return to their home planet, Kinmoku, so that they can restore it. Before she leaves, Princess Kakyuu thanks Sailor Moon for everything that she's done. Then she and the Starlights leave the Earth as four shooting stars that streak across the night sky.