My Past is Not Today

[Jorlem]

Sunset Shimmer. She was the villain of the first Equestria Girls movie, and one of the protagonists of the second.
Edit:
There were two other music videos released alongside this one.  This one is my favorite, but if you are interested, I posted all three in the pony thread over in Other People's Fanfiction.  (I'm probably going to make a thread for the other one I like once I come up for a good power to link with it, and maybe one for the Runway song, but I'm kinda meh on that one, even though it has a fairly obvious utility power.)
Edit 2:To go into more detail on Sunset Shimmer,

Spoiler

she was the personal student of Princess Celestia, before Twilight Sparkle (the central main character of MLP:FiM.)  She was extremely talented, but eventually began to want too much, too fast.  When Princess Celestia refused to accelerate her magic training further, or turn her into an alicorn as she asked (an alicorn is basically a larger, winged unicorn; a pony that possesses the abilities of all three of the main types of ponies), Sunset began secretly studying forbidden dark magic on her own.  When Celestia discovered this and dismissed Sunset from her position as her student, Sunset fled through a magical mirror that led to the human world. 
She returned several years later to steal Twilight's Element of Harmony (a set of six ancient and powerful artifacts.  Twilight's is the central on, the Element of Magic.)  Sunset eventually used the power of the Element to turn herself into a demon, mind controlled the population of a nearby high school, and planned to turn them to demons as well, and use them to conquer Equestria. 
Twilight had pursued Sunset through the mirror, and in the process of trying to locate and reclaim the Element, befriended the human counterparts of her pony friends.  When Sunset tried to blast Twilight with the power of the stolen Element, her friends interposed themselves to try to protect her, which purified the corrupted magic, and turned it back on Sunset Shimmer.

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Stand between the Silver Crystal and the Golden Sea.
"Youngsters these days just have no appreciation for the magnificence of the legendary cucumber."  --Krityan Elder, Tales of Vesperia.