Twilight Sparkle is the primary main character of My Little Pony Friendship is Magic. She is a female unicorn pony who transforms into an Alicorn and becomes a princess in Magical Mystery Cure. She is also the younger sister of Shining Armor and sister-in-law to Princess Cadance. At the beginning of the series, she and Spike, her best friend and assistant, move from Canterlot to Golden Oak Library in Ponyville to study the magic of friendship, and in Twilight's Kingdom - Part 2, she gains her own castle as part of her Friendship Rainbow Kingdom and gains the title "Princess of Friendship". Twilight Sparkle represents the element of magic. Before becoming a princess, she regularly sends friendship reports to Princess Celestia. In Owl's Well That Ends Well, Twilight adopts a pet owl named Owlowiscious.

http://mlp.wikia.com/wiki/Twilight_Sparkle