At first I was trying to come up with an idea for switched identities, sort of like The Prince and the Pauper. However, the more I thought about it, just switching identities didn't seem to provide enough conflict and obstacles for the main characters. That's when I added the brother and sister aspect to the story. Not only do they have to contend with switching identities, but also the problems associated with pretending to be a different gender. This is somewhat more complicated for Willoe and provides for some rather humorous scenes, yet at the same time is critical to both Willoe's and Rowyn's growth.
I also modeled both of them after Emma Stone for their looks and also some of their personality after some of the characters Ms. Stone has portrayed (this more so for Willoe).