The Dark Goddess is Tato's evil counterpart, and 'boss' of all the truly evil antagonists in Dragonball GW, with the possible exception of Ziam, who is basically the Dark Goddess of the Dragonball ER series and is a semi-minor role in Dragonball GW. She is an implied character with several brief appearances (until the Dark Goddess Saga).
The Dark Goddess is very destructive, manipulative, and heartless. For her, there is no such thing as 'family' (SPOILER ALERT: She thinks it makes her weaker.)
(She has all Tato's Techniques) Edit
-People still sometimes refer to Tato as the 'Dark Goddess'.