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.
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.
- Luna, as Nightmare Moon, acted as a villain at the start of The FT Squad's Adventures in My Little Pony: Friendship is Magic, but later became an ally after she is turned back to normal.
- Princess Luna is Hiro's girlfriend.
- Princess Luna is also best friends with Mr. Percival and City of Truro.
- Luna along with Skipper Riley are mentors of Roary the Racing Car and Theodore Tugboat's Adventures team.