Princess Celestia, co-ruler of Equestria, controller of the sun, sister to Luna and wife to Discord. Celestia is the first born of primordial alicorns Helios and Selene — the original rulers of the sun and moon. Under the stern eye of her mother and father, Tia learned how to harness her magic and, someday, take over as ruler of the land. The young unicorn was a fast learner, and eager to please her parents, so everything was going smoothly.
Then, one day, Celestia sneaks out of the castle her parents built squarely in the middle of a dangerous wood. She then finds other ponies, mortals, and watches as a group of unicorns raise the sun. Although her parents were king and queen, they did not trouble themselves with the conflicts of the lower ponies. They ruled purely from a distance. But Tia was very curious, and kept sneaking out to watch the raising and setting of the sun. One day, as the unicorns started the rise of the sun, many of them started seizing and collapsing of magic exhaustion.
In that moment, Celestia decided to step out of the woods and offer her help to her citizens. On her own, Celestia lit her horn and raised the sun through the sky. The ponies around her were shocked to find the filly still standing, and barely breaking a sweat after lifting the celestial body. Tia had earned her cutie mark.
The King and Queen were a little angry at their daughter disobeying their orders, but her cutie mark cemented her place as a future ruler, controller of the sun. Celestia's lessons started becoming more intense and politic-based. Both Helios and Selene were imperialists, conquering and absorbing smaller nations and having them under their control. This is often taught in Equestrian schools as an explaination as to why so many non-equine species live in/around Equestria. Before long, Celestia was ready, ascended, and gained her wings under the eye of her father.
Around this time, Luna gained her own cutie mark, and went into training under Selene. After the ascention of Luna, the King and Queen left, trusting their children to rule the country well in their absence. Their own daughters are not sure where their parents went in the end, but many say the two flew until they reached the edge of the world.
Skip forward a couple thousand years and a couple dozen villain fights later, Celestia has remained the ruler of Equestria. The banishment of Nightmare Moon and by extension Luna, an element bearer, caused Celestia and her sister to lose their connection to them. Thus, Princess Celestia opened her school for Gifted Unicorns, in the hopes of finding at least one of the new bearers. And what luck, as she walked the grounds one day and saw a fully grown dragon burst through the roof of one of the exam rooms.
Celestia has taken a lot of time and effort into reversing what her parents started. Their imperialist rule left many countries in and around Equestria weakened, and Tia did her best to offer peace treaties and offer aid to whoever needed it. What came out of this was a shaky alliance with the Yaks and Dragons, and respectful indifference from the Griffons and Kirin.
Celestia is very proud of her two students Cadence and Twilight, and trusts them and their friends the most in emergencies. As Celestia & Luna are immortal but not invunerable, the two younger alicorns know that they would have to step up if anything happened to either of the sisters. Of course, with all the new alliances with the other countries and the general reverence the ponies hold for their leaders, this may never happen (but better safe than sorry).
Celestia developed a bit of separation anxiety thanks to the whole Nightmare Moon incident, so she likes to visit her fellow princesses whenever she can.
Celestia is currently married to a mister Discord, Draconequus of Chaos (yes that is his legal name), and the pair have three wonderful(ly odd) children. Epiphany is the eldest, and is a studious peace-maker like his mother. He currently works as a neutral party whenever disagreements come up between towns/countries. Delphi, their second born, renounced her crown and title as princess in the process of harnessing her sight seeing powers. She lives deep in the Everfree forest, and lives with Zecora, her mentor. Fairy Floss, their youngest, is a budding artist and mischief maker, a real chip off the old block for Discord. She lives in the castle with her older brother, selling her paintings under a pen name and frequently writing to her BFF Zenith.