Princess Luna

Princess Luna is the younger sister of Princess Celestia and co-ruler of Equestria. While Celestia controls the sun, Luna controls the moon. It is told that long ago, Luna grew bitter and jealous since the ponies, pegasi and unicorns alike flourished and played during the day, but slept during the night. One day, her bitterness got the best of her and she was turned into Nightmare Moon. Celestia tried to reason with her, but it didn't work out, so she reluctantly banished her to the moon for 1000 years, taking responsibilty for both the sun and moon. Princess Luna now enters the dreams of her subjects, mostly children, to assist them in defeating their nightmares.

1000 years later, Nightmare Moon escaped on the Star Summer Celebration and threatened on creating an everlasting night. But thanks to Twilight Sparkle, Celestia's star student, and her five friends from Ponyville using the Elements of Harmony, Nightmare Moon was defeated and turned back into Luna. Therefore, she emotionally reunited with her sister and resumed her place by her side as co-ruler of Equestria.