Baa'lah of Pawn (
www.pawn.se, NOT safe for work!)
EDIT:
Now that I've posted my suggestion, I'll fill in the details.
Pawn is a webcomic drawn by a Swedish artist by the name of Fredrik K. T. Andersson. He's an awesome drawer and is one of the reasons I want to learn arts - I want to be able to create stories like he does in his pictures. Any way, Baalah would be a female demon that has two forms; a size 2 one and identical, much larger form. Neither form is winged. Size 2 form would probably be stealthy and seducer. Also, both forms should be male. She can only seduce women, and be seduced by women.
Shape-changing is an old and respected tradition in comic books, so it goes fine.
Martian Manhunter would also be cool, but he'd be pretty strange with Mind Burn, etherealness (or at least very high stealth/glamour), flying, rejuvenation etc. He'd make for an insane thug, even with his fire vulnerability.