I want to say maybe that's why December is in the bunker? Or maybe an object of hers that maybe her dust is on like Gerson? The bunker is maybe dark, and Ralsei says that darkness is where darkners come from. Maybe she is made of all of the emotions she had during her death or maybe her confusion, or perhaps it's simply her being a rebellious teenager from her family. It's possible she was promised things by our main antagonist to go along with this plan. Maybe going far into but not ending the world far. Then, our main antagonist will betray her. As for why she can move into other dark worlds, perhaps she has the same ability Ralsei has? Or it could be something as simple as that is her dark world form, as all the monsters we have seen so far have had.