Rebecca Mader Rebecca Mader

Rebecca Mader has joined the cast of Once Upon a Time, E! Online reports.

The Lost alum will play a new villain during a major arc in the second half of the season. Further details on her role are being kept under wraps, but viewers will get a hint at her role during the midseason finale, airing Sunday, Dec. 15.

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Since her days on Lost, Mader has appeared on White Collar, Fringe and Drop Dead Diva.

Who do you think she'll play?

Once Upon a Time airs Sundays at 8/7c on ABC.