I found a good quote about Deryn in an interview Scott did:
Of the two leads, Deryn was probably the most fun to write. She's the least introspective character I've even written, preferring action to though any time. So all her scenes have a wonderful momentum to them. She solves problems instinctively, which leads to a very clean writing style. I didn't second-guess myself as I wrote, because Deryn never would either.
She's just fun because she hardly ever
plans anything she just assumes that things will work out. Good thing Alek is a planner. XD