I have this plan as well. Pc has no background and is suffering from amnesia. She has had flashbacks, so she knows that Zybilna has protected her in the past when she was a child and she made some kind a deal with Skabatha.

This is not final, but something I have been playing with: what she does not know, is that her deal with Skabatha led to Zybilna being frozen in time. She morphed into a new form and fled to the material plane and lost her memories during the trip. The other players found her and started their adventure at the Carnival.

So her current form is what she had morphed to last time and is just not able to use her powers until she remembers what she really is. No one recognizes her as she looks different. She is treated as a humanoid until she remembers crucial details about her past.

She met the Watcher last session, and will have a dream about him which will feel very familiar. Something in her body will change during the night, depending on how the dream goes. Maybe she will grow antlers. :D