She may not be one of my favorites but she's well-developed and has a three-dimensional personality. Unfortunately, her development had her lose her badassery.

I'm not even a huge fan of this character but even I can admit she clearly changes as the movie progresses. Both her and Elinor do. The scene in the hall towards the end where Merida is ready to go along with her mother's wishes/beliefs, (before she's interrupted) despite everything, is a perfect example of the development - as is accepting responsibility and sincerely apologising for her actions. Personally I like her character a lot more at the end of the movie.

She is the most well developed imo. She clearly changes, while still keeping her primary character traits. Herr personality is layered, and though this opinion is unpopular, unique.

Personally, I think only Tiana is more developed.