While details regarding her character have not been confirmed, Elizabeth Mitchell is believed to be playing a "possibly malevolent" character who has ties to both sisters Anna (Elizabeth Lail) and Elsa (Georgina Haig). She will also have connections to both Once Upon a Time's Storybrooke setting and Frozen's Arendelle. The events of Once Upon a Time's Season 4 story line will take place after the events depicted in Disney's Frozen.
Along with Georgina Haig, who joined the cast last week, and Elizabeth Lail, Scott Michael Foster is playing Anna's beau Kristoff, who is still getting used to life in the castle after a life spent outdoors.
Elizabeth Mitchell most recently starred in the NBC drama series Revolution, which was cancelled after two seasons by the network in May. The Once Upon a Time gig marks her return to ABC, after starring on Lost and V.