Sunset Shimmer

"Sorry it had to be this way... princess."

- Sunset Shimmer to Twilight Sparkle

Sunset Shimmer is a female unicorn who appears as the main antagonist in the movie My Little Pony: Equestria Girls.

Personality
Sunset Shimmer is described as cruel and dishonest. She treats the other students badly, uses underhanded tricks in an attempt to make Twilight lose the Fall Formal, and drives apart friendships for no apparent reason. She is impatient and power-hungry, intending to use Twilight's crown to rule Equestria.

Despite this, Sunset Shimmer is the least evil of the five main antagonists (below Nightmare Moon, Discord, Queen Chrysalis, and King Sombra), as she is not violent, refusing to kill Spike and letting him go. However, when she is turned into a demon, this changes. Her dark side completely dominant, she becomes violent, destructive, and murderous.

After being returned to normal, either due to her evil side being removed or in horror at what she became, her personality changes again, as she shows remorse for her actions and is willing to let her former enemies teach her about friendship.

Prior Equestria Girls
Sunset's backstory is explored deeper in the comics. Here, it is shown she had a very troubled relationship with her mentor Princess Celestia, which explains her descent into evil. She had previously appeared in the form of a toy and as a NPC in the MLP Gameloft game. In both versions, she was merely a neutral character and not a truly evil one.

Depiction in the film
She is an unicorn pony who steals Twilight Sparkle's element of magic crown, and in a tussle with Twilight, the crown is thrown into a mirror that acts as a portal to another world, which Sunset goes through to get the crown. In this other world, she is depicted as a human who acts very disrespectful and mean to Twilight when they first encounter each other. In the past she drove the human counterparts of Rainbow Dash, Fluttershy, Applejack, Pinkie Pie, and Rarity apart, but Twilight reunites them. She uses underhanded tactics to humiliate Twilight and keep her from becoming Princess of the Fall Formal, but Twilight still easily beats her in the competition. However, she steals the crown from Twilight, puts it on and transforms into a demonic-looking winged humanoid. In this form, she becomes pure evil. With her new powers, she plans to use them and her army of mind controlled teenagers to conquer Equestria. She tries to kill Twilight, but Twilight and her friends use the power of the Elements of Harmony to return her to human form. She is reduced to tears an apologizes for everything that she has done, and Twilight believes her friends can help teach her the magic of friendship. Afterward, the human counterpart of Princess Luna, Vice Principal Luna, hands her a masonry trowel to repair all the damage that she caused.

Powers and abilities
Throughout most of the film, Sunset Shimmer is in human form and devoid of magic, relying on manipulation and bullying to get her way. Her abilities as a unicorn are never demonstrated, but since she was Princess Celestia's personal student, it can be assumed they would be approximately on the same level as Twilight Sparkle's (before the latter became a Princess). Because of her Cutie Mark and her color scheme, it can be theorized she has an affinity to fire magic.

When Sunset Shimmer uses the Element of Magic, she turns into a powerful, winged demonic entity with a fiery look. In this form she displays immense magical powers, despite being in a magicless world. She is proficient with both Dark and Fire spells. She is able to effortlessly destroy walls, turn associates into demons, brainwash large groups of people at once, and conjure powerful fireballs. Her only weakness in this form is that the Element is not "loyal" to her, since she does not grasp the magic of friendship. When she tries to kill Twilight Sparkle and her friends rush to her rescue, the magic of the Element refuses to harm them, instead deserting Sunset Shimmer and imbuing the friends with its power, despite Shimmer still wearing the crown.

Trivia

 * The way Sunset Shimmer is defeated is similar to Nightmare Moon. It is even possible that her demon form is the embodiment of her evil side, much like Nightmare Moon is considered the embodiment of Luna's jealousy, exorcised by the Elements of Harmony and purifying both (though it's also believed that the cause of Sunset's redemption was horror at what she became). It also was a homage to the first defeat of the villain in the series.
 * Sunset Shimmer's name is a synonym to that of her enemy, Twilight Sparkle.
 * Before her redemtion, she shared similarities with all of the main villains in the series so far : Nightmare Moon (relationship to Celestia, defeated in the same way), Discord (breaks friendships for the fun of it), Queen Chrysalis (manipulative, power-hungry, resorts to brainwashing), King Sombra (uses evil magic, can brainwash many people at once, strives to conquer Equestria), and Trixie (arrogant, insufferable, uses the power of a magical artifact that gives her immense power at the price of her sanity).
 * Rebecca Shoichet, who does Sunset Shimmer's voice, also does the singing voice of Twilight Sparkle
 * Her mane/hair (and tail, when she is a pony) look like ketchup and mustard