Sunset Shimmer

Sunset Shimmer, as a human in the human world, acts very disrespectful and mean to Twilight Sparkle when they first encounter each other and is the main antagonist of the new movie My Little Pony: Equestria Girls.

Prior Equestria Girls
Sunset's backstory is explored deeper in the comics.Here,it is shown she had a very troubled relationship with her mentor  Princess Celestia,which explains her descent into evil.She had previously appeared in the form of a toy and as a NPC in the MLP Gameloft game.In both versions,she was merely a neutral character and not a truly evil one.

Depiction in the film
She is an unicorn pony who steals Twilight Sparkle's element of magic crown, and in a tussle with Twilight, the crown is thrown into a mirror that acts as a portal to another world, which Sunset goes through to get the crown. In this other world, she is depicted as a human who acts very disrespectful and mean to Twilight when they first encounter each other. She uses underhanded tactics to humiliate Twilight and keep her from becoming Princess of the Fall Formal, but Twilight still easily beats her in the competition. However, she steals the crown from Twilight, puts it on and transforms into a demonic-looking winged humanoid. With her new powers, she plans to use them and her army of mind controlled teenagers to conquer Equestria. Once she is defeated by the main characters, she returns to her human form. She is reduced to tears an apologizes for everything that she has done, and Twilight believes her friends can help teach her the magic of friendship. Afterward, the human counterpart of Princess Luna, Vice Principal Luna, hands her a masonry trowel to repair all the damage that she caused.