Chibiusa (Chibiusa or Rini in the English versions), is one of the central characters in the Sailor Moon metaseries. When she first appears, she is a small child from the 30th century who visits the past to seek help from the series' primary heroines, the Sailor Senshi. She later returns, a few years older, in order to train as a Senshi herself—Sailor Chibi Moon (called Sailor Mini Moon in the English anime).
Chibiusa is formally named Usagi Small Lady Serenity (Usagi Small Lady Serenity?), but in the 20th century she goes by the name Usagi Tsukino. She is given her nickname by Mamoru Chiba in the manga and Usagi in the anime in order to differentiate from the older Usagi Tsukino, Sailor Moon. The nickname is a combination of chibi (meaning runt, dwarf, shrimp) and her given name, Usagi. In her own time period, she is always called Small Lady. In the English-language adaptations, where Usagi had been changed to Serena, the diminutive "Rini" was used for Chibiusa's name.
Sailor Mini Moon