~Alechsa~
This was actually answered a few times on different threads
3nodding In the anime, ChibiChibi is a personification of Galaxia's Starseed.
In the Manga, she's her own person and is a chibification of Sailor Cosmos, the guardian of the Galaxy Cauldron, the place where all starseeds are created. She failed to protect it from Chaos in her time, and went back in time to teach Sailor Moon that Galaxia had to be stopped.
Some people theorize that in the manga, Cosmos/ChibiChibi is a reincarnation of Usagi, since they have the same abilities and similar looks. However, Kakyuu has healing abilities as well, so it's a weak theory.
In the Manga...
That's not quite true about Sailor Cosmos being guardian of the Galaxy Cauldron as she never stated that she was.
However there's a fairy like entity that lives within the cauldron called Guardian Cosmos who *is* the Guardian.
Sailor Cosmos merely says that she ran away from a battle with Sailor Chaos in the very distant future. <<;;
It's believed that Cosmos is an incarnation of Usagi in the distant future, but it's uncertain on how that came about.
Anime(and in some Musicals): Yeah, Chibi Chibi is the Light of hope that was Sailor Galaxia's Starseed. You see it when they introduce the next arc in stars where she just stabbed the ground and there's all thos pretty petals floating around and she says 'Fly away my light of hope.' >>;;