Chibi Usagi (Chibi Usa for short) is the future daughter of Neo-Queen Serenity and King Endymion. Usagi and Mamoru treat Chibi Usa with parentally love even though neither of them know her true background. Chibi Usa first appears in Sailor Moon R. Travelling from the future, her mission was to find Sailor Moon, in an attempt to save her mother who was sealed in crystal by the Black Moon family. Sailor Moon's identity was unknown to Chibi Usa so the task of finding her proved to be difficult. However, she later discovered the true identity of Sailor Moon and the other Senshi.
CHIBI MOON. Chibi-Usa comes back (Sailor Moon S) to train as a Sailor Scout, taking the name Sailor Chibi-Moon. Her attacks are basically just to annoy the living daylights out of the enemies. She befriends Hotaru, who turns out to be the Senshi of Destruction aka Sailor Saturn who was possessed by the evil Mistress 9. The SuperS series is focused upon her and Pegasus, who hides in her dreams. In the beginning of the Sailor Stars series, she goes back home with her guardian senshi, the former Amazon Quartets, now called the Asteroid Senshi. That's only in the manga, though. In the anime, she goes home alone, and the only thing you know about the Amazon Quartets is that they were healed.