[Huginn]

The girl is most probably a Huldra, a kind of shapeshifting female troll from scandinavia. The huldra could take a human shape, usually a very beautyful one, but her inhuman nature was always betrayed by her tail. ( and sometimes her hollow, treetrunk-like back. But i hope they skip that part).
Or maybe she is a huldrebarn, a changeling, left in the crib of some unsuspecting family, and now at adulthood she discovers that she might be more than a human.

So, yes, i think there will be some TF contend in this movie.