Sunset Shimmer began the Equestrian Girl series as a run of the mill teen movie bad girl. Wanted to be prom queen, dated the protagonist's crush, so on, so forth.
However the writers of the show quickly realized that this did not work so they changed her almost immediately to not just a very likable protagonist, but into a very strong protagonist, who quickly became a part of the mane 6(human 6)which is where the term mane 7 comes from in those movies. She became the glue that held the series together.
Only thing that I regret is that terrible demon design in the 1st movie.....oh god.