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.

Photo Gallery: Celebs we've lost this year

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.