I thought it was clear that it was Elsa. (Sorry. I'm not using spoiler tags. The show has been on, and people shouldn't be here if they don't want to be spoiled.)
I will be surprised, though, if they are bringing her in as a dark character so soon after Frozen. I realize that nothing is sacred as shown by the Peter Pan storyline, but Peter Pan wasn't a currently hot Disney property. That cartoon has been around for half a century (which makes me feel really old).