Princess Luna

Not a whole lot is known about Princess Luna's history aside from what is written in books. What is known is that Princess Luna became jealous when ponies were sleeping through her beautiful night. Her bitterness turned her into Nightmare Moon, and her sister Celestia, along with the Elements of Harmony, banished Luna permanently into the moon. Celestia then took responsibility for raising both the sun and the moon, maintaining harmony in Equestria. For a while at least.

After 1000 years, Twilight Sparkle discovered the return of Nightmare Moon, and with the aid of her friends, fought back against the evil inside of the Princess Luna, returning her to her original form, and ultimately to her sister's side where she once again resumed her job of caring for the night, sleeping ponies, and the dreams of all those in Equestria. While often isolated from other ponies, the readjustment back to pony life is difficult for her, often using her royal Canterlot voice where inappropriate, often scaring those around her, but she proves to be an invaluable asset to both her sister and her kingdom when fighting against Discord, Tirek, Sombra, Chrysalis, and sometimes even her past self.

Luna, along with her sister finally had decided to give up the mantle of leadership in favor of Twilight, Rainbow Dash, Rarity, Pinkie Pie, Fluttershy, and Applejack, but when Tirek made his return decided to put off her retirement to help lead the 6 in their journey to becoming the princesses they were meant to be.

Much like her sister, Luna can show a playful, if less pranky side. She likes to interact with her subjects, especially foals, and feels the most compassion and connection towards them. At the same time she can be very rigid and seem callous, but that is usually self reflection more than an outward projection. She tends to focus a lot of her energy inwards, mostly because she spends a lot of her time alone, as that is when she is awake.

Luna stands taller than other ponies, only Celestia being of similar size, she's a dark navy blue with cerulean and lavender in her mane and tail, with sparkles adorning both. She wears a necklace and crown that match black cutie mark. Four filigreed horseshoes in a light lavender than match her hair, and have a similar shine. Her eyes sparkle teal.

The defeat of Nightmare Moon

Understanding that ponies like to be scared

Saving Starlight Glimmer from Daybreaker