Chibi chibi (which roughly translated means really tiny) is a very small child who one day falls out of the sky just like Rini. She has a little umbrella that she just kind of floats in with that she always carries around. Her very limited vocabulary is "Chibi Chibi!!!" hence the name. She ends up staying with Serena.
In the beginning this character was totally shrouded in mystery and there was much speculation as to her origin. Many put 2 and 2 together and realized that her hair color was close to Rini's, and she had a similar hair style to Serena. Since we found out that Rini was Serena's child from the future could it be possible that Chibi Chibi was Rini's child from an even farther future?!? No!
Chibi Chibi keeps popping up throughout the latter half of Sailor Stars but little is revealed until the very end. We finally find out her origin and why she was around. Since this wasn't revealed until the LaSt episode it is a big spoiler so I put it on another page. If you don't care click this link but be forewarned- it's info from the last episode.