Both!
Sailor Galaxia is a Sailor Senshi determined to steal all of the
Starseeds in the universe, so that she can become the supreme
Senshi.
In the anime, she is possesed by Chaos, and is later cleansed by
Sailor Moon.
In the manga, she's just plain evil, and is used as a pawn by
Chaos. Galaxia is killed, when the purity of Sailor Moon causes
Galaxia's bracelets to break.