Chibi-Chibi has two different origins. In the anime, she was Galaxia's star seed, "The Light of Hope," which Galaxia sent to Earth before Chaos took her over. In the manga, she was Usagi's fighting spirit, Sailorcosmos, and in her older form, looked very much like Usagi. In both versions, she dropped in out of nowhere with an umbrella; in the manga she was at a park, then went to the Tsukino residence, but in the anime she appeared to Usagi first. She hangs around the Tsukino residence until leaving at the end of Stars.
In both the anime and the manga, she usually only says "Chibi chibi" alot [how she came to be called Chibi-Chibi], but can repeat what other people say as well. She seems to really enjoy sweets and playing with dolls. She acts just like a typical little kid.
In the manga, I've heard on pages that she can speak on her own, but I'm not sure how accurate the information was.
Chibi-Chibi wears her magenta hair in two heart shapes on either side of her head. She has bright blue eyes and is very short. She dresses in little kids' clothes. In Sailorcosmos form, her hair is white, and she has hair styled like Usagi's except the "odango" are heart shaped instead of round buns. Her eyes are silver, and she looks to be tall from the picture I have of her.
As Chibichibimoon, I don't think she has any powers, but I could be wrong. She does, however, turn into a sword in the anime [ep 200]. Her fuku is light blue, with magenta bows, puffed white sleeves and bands of gold and magenta under them, short gloves with magenta and gold bands, and those bands again at the top of her skirt. Her white highheeled boots have a magenta band with a big pink heart on them at the top. She has magenta heart shaped covers over her odango, and wears heart stud earrings. She has a strange staff of sorts in the manga, and I've seen her with white wings.
As Sailorcosmos, her costume is very elaborate. Her fuku is white, no bows. On her chest is a gold sunburst shape [also on her choker] on a white circle, framed on either side with pearls and transparent wings. Her sleeves are all gold, and they have pearls with small petals coming off them, also wearing those in her hair. Her skirt has the same piece that's on her chest with two long, thin white ribbons hanging down, and the center pleat is about the size of three, with rainbow colored pieces radiating out from the top. She wears a long white cape, white high heels with the pearls/petals on her inner ankle area, a sunburst on a string of pearls instead of a tiara, and pearl framed white circles covering her odango. She carries a strange staff with her. I can't tell what her earrings are. She has a pearl on each finger, from what I can tell.
This is one cute kid! She's just soooo sweet, and the whole "Chibi chibi!" thing is amusing. She does get old after awhile, though. I like the Sailorcosmos form, and how in manga #18, she actually seems pissed at Galaxia! :) Overall, she's interesting sometimes, otherwise annoying.