Mutant Power: I can't find anything defining them specifically, but she seems to have similar powers to the Scarlet Witch (hexing, etc.), in addition to some illusionary stuff...
Voice Actor: Pauline Newstone (also did the voice of Airazor in Beast Wars)
Guest Appearances: "The HeX Factor" and "Impact"
Back Story: Very little given. Apparently, she's been in cahoots with Mystique in the past, as she's referred to as an "old friend", but anything beyond that is anybody's guess. She's still available via the Brotherhood if anyone needs her, and will do the occasional odd job even for the X-Men if it involves someone she is friends with.
Personality: During the two episodes she was in, Agatha had a very small part- only about five or six lines in each, in fact. So, very little is known about her personality. She's apparently a very patient woman (she'd HAVE to be, to train Wanda), but prefers to keep to herself when she's not needed. And...er... that's it.
Final Reflections: To be honest, Agatha Harkness was little more than a plot device and a fan nod in the series. I can't really seeing there being much more to her character, so she's pretty much near the end of the list when it comes to characters I'll miss.