Chibi Chibi was the civilian identity of Sailor Chibi Chibi Moon. Her appearance was that of a very young child, with red hair set in two heart-shaped odango, and blue eyes. One day, she mysteriously appeared at the Tsukino family home and somehow made Ikuko Tsukino believe that Chibi Chibi was her daughter, similar to what Chibiusa had done on her first appearance. Only Usagi's memory was left intact.
Chibi Chibi's vocabulary was limited to a few simple words (the most frequently used was "Chibi" (ちび), meaning "little" or "mini," hence her name) and repeating what other people said.
In the Stars arc of the manga, Chibi Chibi first appeared floating down to Earth with her umbrella. As she arrived at the Tsukino house, Ikuko was cleaning the kitchen, puzzling over a cup that said "Chibiusa" on the side, wondering whose it could be. When Chibi Chibi rang the doorbell, Ikuko answered the door and, surprised, asked if she was Chibiusa. The girl echoed, "Chibi...chibi?" which became her name.
In the manga continuity, the girls knew that Chibi Chibi could not be Usagi's second daughter, but still wondered who she could be. As it turned out, she was the disguised form of Sailor Cosmos, who was there to guide Sailor Moon to make the decision to destroy Chaos.
In the anime, Usagi ran into Chibi Chibi in the park when the girl's umbrella blew away in a gust of wind. After she returned the umbrella, Usagi left the park but then encountered Chibi Chibi again on the street, but discovered that the girl would only say the words "chibi chibi." She and Luna wondered where the little girl's parents were, but when they looked away for a second, she vanished. When Usagi returned home shortly afterwards, she found the girl there, and her mother scolded her for going out without her "little sister."
Because Chibi Chibi was unable to answer their questions or explain where she came from, the Sailor Senshi speculated that she might be Usagi's second daughter, or possibly Chibiusa's daughter. Setsuna, however, told them that Neo-Queen Serenity only had the one daughter.
It was later revealed that Chibi Chibi was in truth the Star Seed of Sailor Galaxia, and the Light of Hope that the Sailor Starlights were searching for in order to defeat her. In the long ago past, after Galaxia sealed Chaos within her body, she could feel it beginning to corrupt her, so she freed her Star Seed into the galaxy in search of someone who would accept her. Upon reaching Earth, she took the form of Chibi Chibi in order to be near Sailor Moon.
In the final battle against Galaxia, Chibi Chibi revealed the truth about herself to Sailor Moon, and transformed into the Sword of Sealing. She begged Sailor Moon to use the sword to seal away Galaxia and Chaos, but Sailor Moon refused to fight, and the sword was shattered; it transformed back into Chibi Chibi's lifeless body before she vanished.
After Sailor Moon freed Galaxia from Chaos' power, Chibi Chibi reappeared and thanked her, then disappeared.