Chapter 255 was just released and it is confirmed! The Girl is Mavis Vermillion!
EDIT: It's like, Hiro Mashima decided to summon Acnologia and have Natsu defeat it but change his mind at the last minute and have it defeat him instead. He was then thinking of a way to save them and decided to reincarnate Mavis into a Wendy-look-alike who will mysteriously appear and disappear after casting the last legendary fairy magic, fairy sphere. Lolx