Princess Celestia

Princess Celestia is a pegasus unicorn pony with a pinkish-white coat with a sun cutie mark and a long flowing mane and tail of rainbow color. She is the supreme co-ruler of Equestria with her sister Princess Luna, and mentor of Twilight Sparkle, whom she addresses as her faithful student.

Personality
Princess Celestia acts as a mentor to Twilight Sparkle. 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 Sparkle 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.

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 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 Sparkle 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 Friendship is Magic, part 1 tells that one thousand years ago, the day and night cycle of Equestria was ruled by two sisters: Princess Celestia, who brought the day, and younger Princess Luna, who brought the night. However, Princess Luna grew jealous of her sister: the ponies of Equestria all enjoyed and loved Celestia's daylight, but were 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 Princess Luna to become Nightmare Moon, and prevent Princess 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 Princess 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 Sparkle 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 Princess Celestia at the celebration.

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

Present days
Princess Celestia remains the co-ruler of Equestria, alongside Prrincess Luna. She is often seen attending major events or functions, such as the Running of the Leaves and the Best Young Flyer competition.

When Twilight Sparkle was young, Princess Celestia witnessed her rare magical talents and took her as a protégé. After sending Twilight Sparkle to Ponyville and indirectly teaching her about the Elements of Harmony, she instructs Twilight Sparkle 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 Sparkle 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 Sparkle'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 Pie's twitchy tail predictions.

Trivia

 * Princess Celestia and Princess Luna were both based on the Generation 1 Princess Ponies from 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. Princess Celestia is the highest authority, with no king or queen who outrank her.
 * Princess Celestia's name comes from the word "celestial," derived from Latin "caelum," meaning "sky" or "Heaven."
 * Despite being pinkish white in the show, Princess Celestia has been bright pink in every toy release. A possible reason is that the retailers requested it in order to appeal to the little girls.
 * In the game Adventures in Ponyville, when the player plays Rainbow Dash's Cloud Bump game as a unicorn, the unicorn will look like Princess Celestia.

Appearances
Princess Celestia first appearance was in the book that Twilight Sparkle 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. However, 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 appearances goes as follows:
 * S1E2: Friendship is Magic, part 2
 * S1E3: The Ticket Master (In Rarity's fantasy)
 * 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
 * S1E23: The Cutie Mark Chronicles
 * S1E26: The Best Night Ever