A Fan Theory by Samantha Holbert
Could the Wicked Witch of the West be the child of Young Cora and soon-to-be-King Leopold?
We know from various spoilers that Leo did not want an arranged marriage. My theory is that he met The Miller’s Daughter and perhaps fell in love with her. But Cora, being a peasant, and we, knowing what became of her; obviously it did not work out.
When we met Cora in “The Miller’s Daughter,” Young Eva tripped her, and succeeded in humiliating her in front of royalty. It is also obvious that Cora may want power, but she has nothing but contempt for those above her status.
Then there is the deal with Rumple for her first born child. Cora had no qualms about signing that contract because she had already hidden away her first-born; thus duping Rumple.
So the Wicked Witch is the daughter of Leopold and Cora. This makes her both Snow White and Regina's half-sister!
What made her wicked (and green), remains to be seen.