Princess Luna is a prominent character in 'My Little Pony: Friendship is Magic.' She is the younger sister of Princess Celestia and co-ruler of Equestria. Known for her dark blue coat and ethereal mane resembling the night sky, Luna is responsible for raising the moon and guarding the dreams of ponies. Initially introduced as the villain Nightmare Moon, she was later redeemed and plays a vital role in maintaining harmony. Luna often struggles with feelings of loneliness and seeks acceptance from her subjects, reflecting her complex and evolving character within the series.