I have just realized that on "Portraits Page 2" 5th row from top, last picture is "new" or at least to me. I spent some time making these all avatars for my site, and have no memmory of seeing that one at all.
I don't know if anyone has mentioned this, but the lack of Azmodan and the presence of previous-modan seem to indicate to me that he's just placeholding. Don't know why they would be using the old art as a placeholder, but Azmodan is clearly an important boss.
All resemblances aside, I actually don't have any idea exactly who she is. But take a good gander at her second boss window artwork, she's doing something to an old man. Maybe she's the one responsible for turning Cain on us.
I can't wait to hear what his combat dialogue will be.... Maybe something like, "Stay a while and DIE!" I'm not good with catchy antagonistic one-liners. :C