Board Thread:Spoilers!/@comment-2153169-20140913181317/@comment-5154975-20141015141855

Looking at the episode info, I can make a guess and think that the unknown female character appearing is a sister of the Snow Queen and the Queen of Arendelle, as it states that she is "the middle sister of two sisters, one older (Snow Queen) and one younger (Anna and Elsa's mother". She might play a small role and play mediator between her two sisters.