Twilight Sparkle is a purple unicorn and one of the primary characters in "My Little Pony: Friendship Is Magic." She has been a bookworm from an early age, and has such great magical ability that Princess Celestia chose her as an apprentice. Celestia sent Twilight to Ponyville to learn "the magic of friendship," which was actually a plan to reunite the Elements of Harmony and defeat Celestia's corrupted sister, Luna, when she escaped from her prison. After purging the evil from Luna, Twilight requested that she stay in Ponyville with the friends that helped her defeat Luna. Celestia agreed under the condition that she receives regular reports about the lessons learned from their friendships, and set up a library where Twilight and her assistant could live and work. Twilight is fanatical about organization, going as far as to make lists to help her keep track of her other lists. She tries to do things "by the book" and has a plan for nearly anything, but she has difficulty dealing with situations that didn't happen the way she thought they would.
As of season 3, she has ascended to princess status and became an alicorn.
