Celestia and her sister Luna were the former users of the Elements of Harmony and had both been alive for a long time. They banished King Sombre for enslaving the crystal ponies by sealing him deep in the arctic. They sealed Discord for torturing pegasi, unicorns and earth ponies by using the Elements of Harmony. Celestia represented Generosity, Kindness, and Magic when she first used the Elements to defeat Discord.
Few years after Discord was sealed in stone, Luna became bitter and resentful of her sister and their subjects who played in the day but slept throughout the night. Soon the bitterness in Luna transformed her into Nightmare Moon who vowed to bring eternal night. Celestia and her sister fought one another in their castle until Nightmare Moon bested her. With no other choices left a tearful Celestia was forced to use the Elements of Harmony to banish her little sister to the moon for one thousand years. With her sister gone Celestia took on the responsibilities Luna had and eventually left the Elements of Harmony in her old home and went to Canterlot to begin anew. As the years went by she offered the Apple Family who visited her kingdom looking for a suitable place to settle down. An open area close to the Everfree Forest, which was later named Sweet Apple Acres. Sweet Apple Acres brought many ponies throughout the land close to the orchard bringing in more citizens that eventually the outskirts became Ponyville.
The Fall of Sunset Shimmer
Celestia began seeking young students to mentor with potent magical abilities. Eventually, she chooses a young mare named Sunset Shimmer. Sunset Shimmer was a prodigy at magic but was quite boastful and condescending toward others, despite Celestia's lessons about humility. Celestia shows her the "Crystal Mirror" to which Sunset Shimmer became obsessed with it. In her reflection, Sunset saw herself as an Alicorn that could rule Equestria and later something more in the mirror, to which Celestia guided her away from the mirror and told saying it was probably a trick. As the weeks went by Sunset Shimmer constantly asked about the mirror, but Celestia said she'll learn about the mirror when she is ready. However not listening to her teacher Sunset Shimmer began researching the mirror behind Celestia's back in the Dark Magic section of the library and learned the "Crystal Mirror" leads to a different dimension.
However, the librarian alerted Celestia of what Sunset Shimmer was doing. Sunset berates her teacher for hiding such magic from her and demands to be made a princess saying she deserves to stand beside Celestia as her equal or even better. Celestia refuses, saying that being a princess must be earned, and removes Sunset from her position as her student. She has her royal guards escort Sunset Shimmer out of the castle and hears her say "this is the biggest mistake you'll make in your entire life." To which Celestia says "one of many" while looking at the picture illustrating the day she banished her sister to the moon. As Celestia left the library, she discovered the room to the Crystal Mirror open and her guards unconscious. As they awoke, they told her Sunset Shimmer overpowered them and only jumped into the mirror. Celestia later has the mirror moved to her throne room, to remind her and so Sunset Shimmer might come back one day.
Princess Celestia is an alicorn pony and ruler of Equestria alongside her younger sister, Princess Luna. In addition to her responsibility to raise the sun, she also mentored Twilight Sparkle, whom she often referred to as "my most faithful student." She has a female pet phoenix named Philomena.
Princess Celestia was inspired by Majesty, a G1 Unicorn. The name of Celestia comes from the word "Celestial", derived from the Latin "Caelum", which means "sky" or "heavens". Lauren Faust commented that Princess Celestia was originally developed to be Queen Celestia, but her status was changed to "Princess" at Hasbro's request. Lauren Faust also mentioned that during the casting hearings (in the original version), Trixie's voice actor Kathleen Barr did very well during the tests for Princess Celestia until Nicole Oliver took over.
Celestia is larger than any other pony, except for Nightmare Moon, which is almost the same size, until she returns to be Princess Luna. As with the other unicorns, a magical glow surrounds their horn and the objects it is magically affecting. The color of this brightness varies: for example, blue in A Bird in the Hull, orange in The Cutie Mark Chronicles and white in The Queen of Jokes. In The Return of Harmony - Part 1, it's blue when Celestia opens the vault door, but pink when she levitates the empty box of Elements of Harmony that were supposed to be inside her. The brightness is yellow in the opening theme in Lesson Zero, as well as in the episode itself and in all episodes of the second season that follow. Its tail and mane have a natural flowing motion, even when Celestia is not moving.
She has a sweet tooth for cakes. She has many formalities, especially towards dealing with prophesied destinies, but despite this, she does love a good joke and feels bored living in Canterlot. Celestia is caring with all her subjects and truly desires the best for them.
She was like a mother figure for Twilight and always believed in her student's potential.