Once Upon a Time has cast a pair of key Season 4 roles.

According to TV Line, relative newcomer Elizabeth Lail will come on board the ABC series this fall as Anna, Queen Elsa of Arendelle’s younger sister in the film Frozen; while Greek alum Scott Michael Foster will play Kristoff.

These are two of the three Frozen characters set to make regular appearances on Once Upon a Time Season 4.

The third, of course, will be Elsa herself. She remains uncast.

The role of Anna - described in casting notes as someone with an unbreakable spirit who often gets into trouble - marks the first steady appearance on the small screen for Lail.

Kristoff, meanwhile, is outlined as someone who has been sleeping outside for years when we first meet him and also someone deeply loyal to both Anna and Elsa.

In addition to his run as Greek‘s Cappie, Foster’s television credits include The River, Californication and AMC's Chasing Life and Halt and Catch Fire.

Once Upon a Time Season 4 will kick off in September.

UPDATE: The show has now also cast Elsa!

Matt Richenthal is the Editor in Chief of TV Fanatic. Follow him on Twitter and on Google+.