Character: princess celestia
Princess Celestia is an Alicorn with a white body. Her long, flowing mane and tail come in various pastel colors, and she wears a golden tiara with an amethyst on her head and a matching necklace around her neck. She is the eldest Princess, between she and her sister Princess Luna, and her duty is to raise the sun, to bring forth the day. Her cutiemark is the Sun.
Celestia is the almighty tallest of all ponies. 'Cause she's da biggest dat makes her da boss of all dose weedy grots. Although there have been no outward examples of religious institutions in Equestria, Celestia seems to be the subject of a kind of worship like a pharoah.
Celestia used to rule Equestria as half of a biarchy with her sister Princess Luna, but
it turns out that Luna was a mmmbitch who didn't like bananas Luna went loony and started calling herself Nightmare Moon. When exactly the biarchy began is unclear, but it would seem to have happened some time after the three tribes joined.
One thousand years before the start of the series, Celestia sealed Nightmare Moon in the moon (BEEYAAAAAAAATCH!) with The Elements of Harmony. She is Twilight Sparkle's teacher and gave her the task to "make friends" in Ponyville upon learning of Nightmare Moon's return. However, after her sister is restored to her true form, she forgives her and allows Twilight Sparkle to remain in Ponyville with a new task of "studying the magic of friendship". She always refers to Twilight Sparkle as "My Faithful Student".
It is unknown exactly how old Princess Celestia is, but by deduction based off the events of the show, she is more than 1000 years old. It should be noted that Celestia does not seem to have aged or changed in any way since she sent Nightmare Moon to the moon, as evidenced by a vision experienced by Princess Twilight while under the effects of a salty, gooey, and white potion provided to her by Zecora in the episode Princess Twilight Sparkle. That would seem to indicate that Celestia is immortal.
Princess Celestia's personality is very aloof and mysterious. Her motivations are often unclear, although her gracious manner makes it easy to assume that she has only the best of intentions. Curiously, Twilight Sparkle occasionally imagines Princess Celestia as the sword of Damoclese ready to fall upon her at the first sign of failure such as in A Bird In the Hoof and Lesson Zero.
We do know that Princess Celestia has an impish sense of humor. She delights in small pranks (see: A Bird In the Hoof) and has been known to subtly manipulate situations to create chaos for her own amusement (see: The Best Night Ever). The IDW "A-Team" expanded on this character trait in their comic series, thus allowing Celestia to have a bit of fun while dimension-hopping with Starswirl the Bearded.
In the afore-mentioned comic, Celestia became romantically attached to the nice guy mirror world version of King Sombra. Celestia demonstrated a reckless passion that has not been seen outside of the comics. It ended in tragedy of a sort with the mirror Sombra becoming consumed by evil.