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.

Seasons 2 and 3
At the beginning of Seasons 2 and 3, it's revealed that Luna, alongside Celestia, turned Discord to stone with the Elements of Harmony and imprisoned King Sombra in ice while turning him to shadow, about a thousand years ago sometime before her transformation into Nightmare Moon.

Nightmare Night
In the season 2 episode "Luna Eclipsed", Princess Luna, now uncorrupted (And with a new look. Lauren Faust, the show's creator, stated that this was because when Nightmare Moon was defeated, Luna was reborn with very low magic levels, and she transformed after getting her good powers back), returns to Ponyville during the holiday Nightmare Night(Equestria's equivalent to Halloween), a holiday dedicated to Nightmare Moon depicting her as a scary bogeyman. Luna, with Twilight's help, tries to change her image from that of the terrifying Nightmare Moon, to a favorable one as Princess. Unfortunately, the nature of the holiday, combined with Luna's outdated mannerisms and flair for the dramatic, make her subjects more scared of her than ever. Her first attempts end disastrously, prompting her to decree Nightmare Night canceled, but Twilight persuades her to give Ponyville another chance. When Luna discovers that the residents actually enjoy being scared as part of the night's festivities, she uses the Nightmare Moon persona to help them have fun and thus gains their trust and friendship.

Season 4
During the Season 4 premiere two-parter "Princess Twilight Sparkle", Twilight attempts to figure out the source of the chaos seizing Equestria by drinking a potion which gives her flashbacks of the past. One of the flashbacks she experiences is of Luna becoming Nightmare Moon and assaulting Princess Celestia. At first Celestia is reluctant to fight her sister, but Nightmare Moon assaults her with dark beams and eventually wounds her, sending her plummeting to the ground. However, Celestia promptly recovers and arms herself with the Elements of Harmony, the power of which she channels into a powerful beam that overwhelms Nightmare Moon's defenses and seals her inside the moon. It's revealed she also aided her sister in defeating Discord a millennium ago.

Hiro
Princess Luna also has a relationship with Hiro the Japanese Engine. It all started when Thomas found Hiro and Princess Luna grew very fond of him. Soon enough, she felt a love for Hiro inside.

So they began to go out, and then one peaceful night, Hiro showed Luna a ring that has.

In [The Night to Remember]], Hiro and Princess Luna are to be married. However, during wedding prep, Luna was captured. So Hiro along with the rest of the gang journeyed out to get her back, on the way to the villains' hideout, they encounters tough guys, a Spinosaurus, Catrina, and soon enough they get help from Star Swirl the Bearded and Scorpan. They all journey back to Canterlot and battle the many villains. After the villains are defeated, Luna and Hiro are finally married.

Later, Princess Luna gave birth of a night Alicorn filly named Princess Yuna in The New Princess. Then, in Revenge of the Deceptitrains, they adopted Snowdrop, since her current mother died.