The younger sister of Celestia, Luna is a winged unicorn who has an indigo body and a teal mane and tail. She raised and lowered the moon each night, but she becomes jealous of Celestia because the ponies sleep through the night and thus cannot appreciate her work. This jealousy eventually transforms Luna into the shadowy Nightmare Moon. As Nightmare Moon, her body is extremely dark purple with light-blue armor. Her mane and tail appearing to be made from sparkly indigo, with purple-rimmed moon dust that she can use as limbs. Celestia banishes her to the moon after she tries to bring eternal night to the world. During the pilot episode, Nightmare Moon breaks free after a thousand years and returns to Equestria on the longest day of the year, intent on carrying out her plan again. She uses her powers to hinder Twilight Sparkle and the other ponies in their search for the Elements of Harmony. Eventually, the Elements transform her back into Luna and she resumes her place at Celestia's side.
Luna returns to Ponyville during Nightmare Night, a festival similar to Halloween, with a tall and slender build, a darker coat, and a translucent-bordered mane and tail. With Twilight's help, she tries to change her public image from that of the terrifying Nightmare Moon, on which the holiday is based, to a more favorable one as Princess Luna. However, because of her outdated mannerisms and extremely loud 'Royal Canterlot Voice', her first attempts are unsuccessful, and she decrees Nightmare Night canceled. But Twilight persuades her to give Ponyville another chance. When Luna discovers that the residents enjoy being scared as part of the night's festivities, she uses the Nightmare Moon persona to help them have fun and thus gain their trust and friendship, along with overcoming her social awkwardness.
Luna returns in The Crystal Empire: Part One. She is seen with her sister, Princess Celestia, as they contemplate wether or not Luna should be sent to the Crystal Empire instead of Twilight Sparkle. She seems a little bit skeptical of Twilight during this scene, as she is denied permission to carry out the mission by her sister. She possesses the ability to enter ponies' dreams and help them face their fears, as she does with Scootaloo in "Sleepless in Ponyville."