Princess Celestia is a white alicorn, and the co-ruler of all of Equestria (along-side her younger sister, Luna). She is also the mentor for Twilight Sparkle, who she sometimes refers to as "Her faithful student" and is shown to have a lose bond with.
Celestia has lived for a very, very long time and has done many things. She has defeated her own sister (as Nightmare Moon), defeated Discord, fought Chrysalis and of course, ruled Equestria, facing its challenges day-in, day-out.
Her cutie mark is an orange sun, and she is usually seen wearing a chest plate, crown and shoes with purple gems incrusted in them.