Sunset Shimmer is Princess Celestia's old student. She wanted to have complete power, and thus, was dangerous. She found this mirror in the castle, and went through it to escape from Celestia's guards. There, was Canterlot High, a human high school. Sunset Shimmer was a bully there, until Twilight Sparkle came. After Sunset turned into a demon-like creature, and possessed Twilight's Element of Harmony, Twilight and her friends defeated her with the Magic of Friendship. After she is defeated, Sunset Shimmer decides to be good, and apologizes.
An image of human Sunset Shimmer that appears on the back of the Equestria Girls Sunset Shimmer & Twilight Sparkle doll release's packaging was posted online in mid-March 2013.
On March 19, 2013, Meghan McCarthy was asked "Is Sunset Shimmer going to be a Season 4 storyline? The fact that her backstory seems like a show plot" and responded "Nope." On April 1, 2013, McCarthy wrote as part of an April Fools' Day joke that My Little Pony Friendship is Magic season four episode five would be titled "The Student Becomes the Teacher: The Return of Sunset Shimmer".
If you would like to RP with Sunset, just mention her! She will RP with mostly everyone, as long as it is appropriate, and if she isn't busy!