Princess Luna

Princess Luna, formerly known as Nightmare Moon, is a Pegasus unicorn and the younger sister of Princess Celestia, and is the main antagonist (as Night Mare Moon) of the first two episodes of My Little Pony: Friendship is Magic. Luna is restored back to her former self through the magic of the Elements of Harmony and returns to rule Equestria alongside her older sister.

Role in the series
According to the Fanfiction series, the Villain League intended to get Princess Luna back into Nightmare Moon for their own nefarious purposes. Also, in the Chronicles series, Batula had a crush on her, and intended to turn her back into Nightmare Moon so he and she could rule the night together. Luna also knows the Shell Lodge after the events of the Fanfiction episode.