Chibi Chibi is the cute little girl that floated down out of the sky on an umbrella. She met Usagi when her umbrella flew away and Usagi caught it for her and returned it. Usagi noticed that the child was following her so she went and asked her some questions. The only answer she got was "Chibi Chibi!" Turning her eyes from the little girl for a second, ChibiChibi disappeared. When Usagi went home, ChibiChibi was there waiting for her. Ikuko-mama scolded Usagi for leaving her "little sister ChibiChibi" (sort of reminds you of Chibiusa ^_^) No one knew where she came from because she couldn't talk. (Well, actually she could, in the manga anyway.) The only thing she said was "Chibi Chibi!" The rest of the senshi assumed that she was Usagi's second daughter from the future. Then Setsuna said that it couldn't be her daughter. (In the manga, it was the Asteroid senshi who said they had been taught that the Millennium queen gave birth to only one princess) So then they assumed that Chibi Chibi was Chibiusa's daughter. Little did they know how wrong they were...
Sailor Chibi Chibi turned out to be a MAJOR force in the final battle with Galaxia and her soldiers. She saved Sailormoon more than once. 
In the manga, Galaxia melted Sailormoon's chalice and ChibiChibi restored it and the city of Tokyo. In the anime, it is revealed that Chibi Chibi is actually the starseed released by Galaxia when she was possessed by Chaos. In the manga, Chibi Chibi is the ultimate reincarnation of Sailormoon from the way distant future, Sailor Cosmos. She carries her own scepter that she uses to block attacks. Once she even deflected Galaxia's attack, the Galactic Super String! Sailor Cosmos ran away from the future when she was defeated by Sailor Chaos. Her future was filled with pain, war, and death. She went to the past, to urge Usagi (herself) into making the right decision to destroy the Galaxy Cauldron because Chaos had merged into it. Eternal Sailormoon refused because without the Cauldron, no one could be reborn. So you see, Cosmos IS Sailormoon! After seeing, Eternal Moon's courage to stand alone and continue to battle with her enemies, Cosmos regained her hope and she too returned to her future to continue her battle against Sailor Chaos. That is the last you ever see and hear of Sailor Cosmos.
