Medusa, from the Inhumans (and occasionally the Fantastic Four).  She was another character who wasn't quite as beautiful when Chic Stone or George Roussos was inking Jack Kirby as she was when Vince Colletta or Joe Sinnott was doing the inking.

And she and The Harlequin over at DC may be the only characters who started out as an adversary of a group or one of the heroes of a group, then later fought aligned with that group.  I always wonder if Kirby and Stan Lee intended her to reform from the start or just decided after creating her that she was too good to be a villain.