Princess Celestia

Princess Celestia is a pegasus unicorn pony with a pinkish-white coat, a long flowing mane and tail of rainbow color that move even when there is no breeze and a sun cutie mark. She is the supreme co-ruler of Equestria with her sister Princess Luna, and mentor of Twilight Sparkle, whom she addresses as "My faithful student".

Personality
Princess Celestia is a wise and kind pony. As a teacher, she often does not directly state her goals, but rather gives the advice to help her students find the answers they need, like her letter prompting Twilight to make friends in Friendship is Magic, part 1 when her goal was for Twilight to find the Elements of Harmony. Though viewed with great reverence and awe due to her position, she often encourages her subjects to relax with jokes and gentle questions, and shows great patience with any rudeness towards her, as she shows to Pinkie Pie when she rudely snatches a cupcake right out of her hoof at a party in A Bird in the Hoof. During the same party, she playfully tricks the Cake couple into overfilling her cup and disarms their nervousness.

While always regal in both appearance and manner, Princess Celestia is shown to be fun-loving and at times will even disregard the strict standards of the "high class" when it becomes too much. In The Best Night Ever, even after her student Twilight's five friends completely ruin the Grand Galloping Gala, Celestia was more than thrilled with their performance as they truly "livened up" the scene, going so far to admit that the Gala is usually unbearably boring, primarily due to the fact that she spends the entirety of it welcoming guests and never actually gets to enjoy it. This playful, almost prankish nature has sometimes caused fans to reffer to her as "Princess Trollestia". This again displays how she's a good sport even when things don't proceed in the stately manner most would expect them to be in her presence.

Skills
Being one of the two pegasus unicorn ponies, Princess Celestia possess magic far beyond normal unicorn ponies. She is able to control the sun and the daytime sky, bringing them out each day and then moving them away to make room for the night. In Princess Luna's absence, she was responsible for moving the moon as well. She instantly calms Twilight and gets her to revert her accidental spells in The Cutie Mark Chronicles. and she is extraordinarily long-lived, as she has been ruling over Equestria for more than a thousand years.

One thousand years ago
The legend narrated in the first episode tells that one thousand years ago, the day and night cycle of Equestria was ruled by two sisters: Luna, the younger, who brought the night, and Celestia, who brought the day. However, Luna grew jealous of her sister: the ponies of Equestria all enjoyed and loved Celestia's daylight, but were all asleep at night. No one was awake to appreciate the beautiful night she had worked so hard to create. Eventually, this jealousy and bitterness drove Luna to become Nightmare Moon, and prevent Celestia from bringing forth the sun. She intended to keep Equestria in eternal night. Her older sister, with great sadness and regret, stopped Nightmare Moon by using the Elements of Harmony, and imprisoned her within the moon. Henceforth she became known as the Mare in the Moon and knowledge of her slowly drifted to the area of myths.

Nightmare Moon's escape
A thousand years passed under Celestia's rule, until the night of the Summer Sun celebration, held in honor of Princess Celestia and the longest day in Equestria. Shortly before the celebration, Celestia's student, Twilight Sparkle informed her of the imminent return of Nightmare Moon. Celestia responded by sending Twilight to Ponyville to oversee an upcoming party, and make some friends. The princess knew Twilight had the power to stop Nightmare Moon, but the young unicorn needed to understand friendship in order to wield the Elements of Harmony. Nightmare Moon escaped as prophesied, and appeared in place of Celestia at the celebration.

After Nightmare Moon was transformed back to her former self, Celestia offered forgiveness to her little sister, and proposed they rule together. With Equestria back under both sisters' rule, Celestia allowed her student to stay in Ponyville with her friends, instructing her to study the "magic of friendship".

Present day
Celestia remains the co-ruler of Equestria, alongside Luna. She is often seen attending major events or functions, such as the Running of the Leaves and the Best Young Flyers Competition.

When Twilight Sparkle was young, Celestia witnessed her rare magical talents and took her as a protege. After sending Twilight to Ponyville and indirectly teaching her about the Elements of Harmony, she instructs Twilight to send her periodic reports on the magic of friendship in the form of letters, which Twilight usually sends via Spike. These letters typically serve to illustrate each episode's moral for the show's viewers.

However, in several cases, Celestia has been present during an episode, and Twilight delivers the report to her in person. Examples include Swarm of the Century and Fall Weather Friends. In Sonic Rainboom, Rarity delivers the report instead, and in A Bird in the Hoof Celestia hears Twilight's intended report from Fluttershy

Celestia makes a surprise cameo at the end of Feeling Pinkie Keen, dropping out of the sky to grab Spike's letter and then taking off just as quickly, confirming another one of Pinkie's "twitchy tail" predictions.

Trivia

 * Princess Celestia, alongside Princess Luna were both based on the Generation 1 Princess Ponies on the 1980's animated series.
 * Princess Celestia was originally supposed to be Queen Celestia, as is logical considering her position, but Hasbro requested the name be changed as Disney has caused princesses to be associated with good and queens associated with evil. Celestia is the highest authority, with no king or queen who outrank her.
 * Celestia's name comes from Latin "caelum" meaning "sky" or "heaven".
 * Rarity's fantasies reveal that Celestia is supposed to have a "nephew". Faust described him a very distant nephew on Celestia's mother's side, and he was later identified as "Prince Blueblood" during The Best Night Ever.
 * Despite appearing pinkish white in the show, every toy release of Princess Celestia has been bright pink in color. A possible reason is that the retailers requested it in order to appeal to little girls. In the Adventures in Ponyville game, when playing Rainbow Dash's game as a unicorn, the unicorn will look like princess Celestia.

Appearances
Princess Celestia first appeared in the book Twilight was reading at the beginning of Friendship is Magic, part 1. Though it wasn't specifically stated that it was her the book was talking about, only that there was an older and a younger princess. She made a full appearance in the next episode. Since then, she is sometimes shown reading Twilight's letters and occasionally visiting Ponyville. The complete list of her apperances goes as follows:
 * S1E2: Friendship is Magic, part 2
 * S1E5: Griffon the Brush Off
 * S1E10: Swarm of the Century
 * S1E12: Call of the Cutie
 * S1E13: Fall Weather Friends
 * S1E15: Feeling Pinkie Keen
 * S1E16: Sonic Rainboom
 * S1E22: A Bird in the Hoof
 * S1E26: The Best Night Ever