Sunset Shimmer

"When the crown and its power are mine, Twilight Sparkle will be sorry she ever set hoof into this world... not that she would've been much safer if she'd stay in Equestria."

- Sunset

Sunset Shimmer (voiced by Rebecca Shoichet) is the reformed main antagonist in My Little Pony: Equestria Girls and one of the main protagonists in its sequel, My Little Pony Equestria Girls: Rainbow Rocks. She also appeared in the IDW comics, where her backstory is detailed. She is a female unicorn pony and Princess Celestia's former student.

Personality
Throughout the first film, Sunset Shimmer is shown to be mean, rude, short-tempered, greedy, bratty, rebellious, egotistical, cruel, condescending, manipulative, and dishonest. She frequently insults and belittles others, and thinks little of it. According to human Fluttershy, Sunset makes life miserable for "anyone who stands in her way". As a result, much of the Canterlot High student body is afraid of her. She is also shown to be mainly concerned with gaining power, desperately trying various attempts to get Twilight's crown in order to use its magic.

However, she doesn't appear to be willing enough to inflict physical pain on others, or at least not upon small animals (i.e. Spike), claiming she isn't a monster. She appears to be easily irritated when someone else is praised in her presence, to the point of being provoked into attacking them.

When Sunset puts on the element of magic, she transforms into a demonic visage of herself. While in this form, she becomes more dangerous and goes so far as attempting to kill Twilight. After she is defeated by the Elements of Harmony, Sunset Shimmer becomes immediately repentant and apologizes for her actions, appearing to renounce her prior dishonest and unkind ways.

In the second film, Sunset Shimmer appears to have completely changed from her old self and started believing in friendship, becoming friends with her former enemies, even Twilight, and friendly toward others. She also appears deeply repentant for her actions made at the end of the first film, referring to her altered self as a "raging she-demon". By the end of Rainbow Rocks, Sunset starts writing friendship reports to Princess Twilight using the book she once used to communicate with Princess Celestia.

Backstory
"I deserve to stand beside you and be your equal... if not your better. Make me a princess."

- Sunset Shimmer to Celestia

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 (though she was already extremely arrogant, self-centered, and antisocial, thus her mentor's short patience with her). Sunset 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.

Sunset's desire was to gain princesshood; however, she felt Celestia's tutelage wasn't getting her anywhere fast enough. During the night, she took it upon herself to do research on the mirror Celestia had shown her some time ago. Just as she found information about the mirror, Celestia and two royal guards arrived on the scene. At first, Celestia tried to console Sunset, but Sunset retaliated by demanding to be crowned a princess.

It was then Celestia saw that Sunset lacked the qualities to be a true princess, and deemed the young unicorn unworthy of such a status. Celestia then expelled Sunset from the school and had the guards escort Sunset out. Sunset told her former mentor that this would be the greatest mistake ever made, but of course it was only one of many. When the chance came, Sunset took the guards by surprise and jumped through the mirror -- emerging as a teenage girl on the other side.

My Little Pony Equestria Girls Movie
"Twilight Sparkle wants to be your Fall Formal Princess. What does it say about our school if we give someone like this... such an important honor?"

- Voiceover in an anti-Twilight Sparkle video

Sunset Shimmer first appears as the main antagonist in the first film. She is an unicorn pony who steals Twilight Sparkle's element of harmony: her magic crown, and in a tussle with Twilight Sparkle, the crown is thrown into a mirror that acts as a portal to another world, which Sunset goes through to retrieve the crown. In this other world, she is depicted as a human who acts very disrespectful and mean to Twilight Spakle 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 Sparkle and keep her from becoming Princess of the Fall Formal, but Twilight Sparkle still easily beats her in the competition.

However, Sunset steals the crown from Twilight Sparkle, puts it on, and transforms into a demonic-looking winged humanoid. In this form, She becomes purely evil. With her new powers, she plans to use them and her army of mind-controlled teenagers to conquer Equestria. Sunset attempts 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 and apologizes for everything that she has done and Twilight believes her friends can help teach her the magic of friendship.

Afterwards, the human counterpart of Princess Luna, Vice Principal Luna, hands her a masonry trowel to repair all the damage that she caused.

My Little Pony Equestria Girls: Rainbow Rocks
"I don't think anyone is supposed to have all the answers. But you can count on your friends to help you find them."

- Shimmer helping Twilight and her friends confront the Dazzlings.

Sunset Shimmer re-appears as one of the main protagonists in My Little Pony Equestria Girls: Rainbow Rocks, and was reformed from her villainous ways in the first film. With the exception of the Rainbooms, the rest of Canterlot High still bears resentment toward her and she even resents herself for the actions she'd taken as what she refers to as a "raging she-demon". She even admits to herself and her friends that she only dated Flash Sentry to make herself more popular.

When Sunset gives the The Dazzlings a tour of the school, she gets a bad vibe from them, when Adagio Dazzle gets defensive about her pendant. Her suspicions are later confirmed when the Dazzlings mesmerize the students and faculty when they are singing. Using a magical book with her cutie mark on the cover, Sunset sends an SOS to Twilight Sparkle in Equestria. During Twilight's role as the Rainbooms' lead singer, Sunset acts in a supportive role and keeps an eye on the competition, especially the Dazzlings during the Battle of the Bands. At one point, Sunset Shiimer confronts the Dazzlings, who try to manipulate her insecurities by pointing out that the Rainbooms are reluctant to allow Sunset Shimmer into the band.

When the Rainbooms lash out at each other during the final round of the Battle of the Bands, Sunset realizes the Dazzlings' true intentions and is able to stop the arguing. She even accepts partial blame due to her lack of knowledge in friendship.

When the Dazzlings overpower the Rainbooms during the film's climax, Sunset Shimmer steps up and sings on the band's behalf. Her help not only gives the Rainbooms the strength needed to defeat the Dazzlings, but she also undergoes a half-pony transformation like her friends. Afterward, Sunset Shimmer joins the band as a singer and guitarist, and she uses her old journal that she communicated with Princess Celestia through to write her friendship reports to Twilight Sparkle.

During the credits, Sunset is seen having now been accepted by the rest of the school, as the Cutie Mark Crusaders walk happily with her and Bulk Biceps helps her get Angel down from where he is stuck.

Sunset Shimmer is featured in the songs Welcome to the Show and Shine Like Rainbows during the film.

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 and arguably so than, turn associates into a demon, 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 tried 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 power, despite Sunset Shimmer still wearing the crown.

Trivia

 * She is the first major female antagonist in the series who is not a monarch, the second being Lord Tirek, and the third the Dazzlings.
 * 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 an homage to the first defeat of the villain in the series.
 * Before her redemption, she shared similarities with most of the main villains in the series so far: Nightmare Moon (past with Princess Celestia, seeking to rule over Equestria, and defeated in the same way by Twilight Sparkle), Discord (breaks friendships for the fun of it), Queen Chrysalis (manipulative, power-hungry, resorts to brainwashing), King Sombra (uses evil magic, seeks to conquer Equestria), and mostly Trixie (arrogant, insufferable, uses the power of a magical artifact that gives her immense power at the price of her sanity). She is also similar to Discord in that one of the Mane Six's actions was able to reform her; Fluttershy for Discord and Twilight for Sunset Shimmer.
 * Rebecca Shoichet, who does Sunset Shimmer's voice, also does the singing voice of Twilight Sparkle.
 * Sunset Shimmer can be considered an evil counterpart of Twilight Sparkle due to how many similarities they share. Some fans have speculated she is what Twilight may have become had she not discovered friendship and its meaning.
 * After starting to believe in friendship, Sunset Shimmer acts more like Twilight Sparkle, being the peacemaker of the group, having insecurities, and developing a worriesom personality. A difference is that Sunset is quite patient with the group's rage while Twilight is quite temperamental with hers.