Years of writing characters wasted!

Bad writing is what makes her a mary sue, which is the case with most mary sues, since hardly anybody actually intends to write one.

Here's why:
She is an important side character we are supposed to sympathize with, and whose narrative is supposed to be important to the story while
BUT:
1) She doesen't undergo any significant development – story ends with her being just as much of a whiny brat as she starts, just with more powers. We are expected to believe she becomes better but we were not shown it, so as far as the viewer concerned, she didn't.
2) She is liked by the rest of the cast despite being written in a very unlikeable manner. The dissonance between cast reaction to her and viewer reaction is a typical trait of MS.
3) She receives rewards and preferences without doing anything to deserve them, but other cast acts like she deserves them.
4) Her negative traits are never paid attention to by the crew aside from Maya.