She is one of the Sailor Senshi in training from the series Bishoujo Senshi Sailor Moon. Her civilian identity is Tsukino Usagi, but to differentiate her from her mother who has the exact same name, she goes by Chibi Usa. A common gag in the anime's third story arc is how Chibi Moon's Pink Sugar Heart Attack doesn't always work properly.
The following tags implicate this tag: super_sailor_chibi_moon and super_sailor_chibi_moon_(stars) (learn more).