Angie being a demon lord seems like the obvious hint here, but at the same time, Belgrieve found her in the mountains in a basket filled with protection charms, and I feel like if she was straight up some kind of demon lord she would be more likely to be disposed of / thrown in the woods without any charms, instead of the kind of "I can't take care of this kid but I don't want it to die" abandonment treatment.
Since we know the previous demon lord talked about being lonely/wanting to meet master, and this one wants to "go back", I'm thinking it might be possible it's kind of a fakeout and she's not "the same" kind of physical entity, it just sensed some emotion in her heart as being the same (either from being abandoned as a child, or from usually not being able to ever meet with her dad) instead.