Chibiusa is the Pretty Soldier Sailor Chibimoon. She is Neo-Queen Serenity's and King Endymion's daughter, from thirtieeth century Crystal Tokyo. She first appeared in the second arc of the R season when she fell from the sky and landed on top of Usagi's head while she and Mamoru were kissing. She demanded the ginzuishou, then later turned up at Usagi's house where she had hypnotized Usagi's family using Luna P to think she was Usagi's cousin. Usagi and the others try to help her after they realize the Dark Moon is after her, so they travel to the future using the time key. Eventually the Dark Moon succeeds in kidnapping her, brainwashing her into Black Lady.
In the R series, Chibiusa is basically a bratty little kid. In S, she returns with a note from Neo-Queen Serenity saying she is going to stay in the past and train as a senshi, Sailor Chibimoon. In S, her attitude changes greatly from R. She is cute and sweet at times, especially when she is with Hotaru. In SuperS, Hotaru is no longer in the series, and Chibiusa is back to being a brat. Eventually the series turns and focuses around Chibiusa (the SuperS movie, too) while she has her crush on Elios. (At least she's off her crush on Mamoru, her FATHER). In Stars she fades away early off after Mamoru is killed, so we can't judge her very much there.
As Sailor Chibimoon, she is pretty pathetic. In S, half the time her attacks didn't work, and when they did she didn't do much damage. In SuperS she used the Twinkle Yell to summon Pegasus/Elios for help, but that was mostly her job.

Chibiusa "Little Rabbit"
English Name: Rini
Element: Purity and Soul
Birthday: June 30
Sign: Cancer
Age: over 900 (manga age)
Blood Type: O
Guardian: Diana/Luna P/Sailor Pluto
Favorite Subject: Drawing
Least Favorite: Language
Goal: To become a lady
Seiyuu: Araki Kae

Attacks

Luna P Henge: As Chibiusa, she says this and Luna P tranforms into something

Moon Prism Power, Make up: Transformation phrase
Pink Sugar Heart Attack: Pink hearts shoot of her wand (doesn't really work)

Crisis Make up: Super transformation phrase
Twinkle Yell: Yells this and Pegasus comes to help
Moon Gorgeous Meditation: attack using the Moon Kaliediscope