To me Sunset is kind of a mirror, but also an idol.
I can relate to her the most out of the franchise, especially to the Sunset in the first and the one in the second movie.
I might not be a school bully and I have never been one, but just like Sunset I want to be the best and can not stand it when someone is better in something than I am...What can lead to sudden aggression outbreaks.
I also do not really respect my teachers and skip school a lot to learn things I am actually interested in and in my own way (I guess, you could count this to "Sunset leaving Celestia"?).
But I am also an outcast like she was in Rainbow Rocks, and in Legend of Everfree I could also realte to her teasing Sci. That is kind of how I am when I am feeling comfortable around people.
But the main thing, why I can relate to her, is her struggle with forgiving and forgetting her past.
The end of the second movie is when Sunset also became an idol to me, even though I am not really interested in making friends.
I hope we will see her in the show sometime.