Sunset Shimmer

History
Sunset Shimmer appears as the main antagonist in My Little Pony Equestria Girls. She steals Twilight Sparkle'selement of magic crown, and during a tussle with Twilight, the crown is thrown into a mirror which acts as a portal to another world. Sunset then follows the crown shortly after.

Princess Celestia reveals that Sunset was a former student of hers who began her studies not long before Twilight did. However, when she didn't get what she desired as quickly as she'd liked, she turned cruel and dishonest, eventually abandoning her studies to pursue her own path. In this other world, Sunset has been living as a human and student of Canterlot High for some time. She discovers that Fluttershy had found the crown and turned it over to Principal Celestia, much to Sunset's annoyance. Upon learning that Twilight will be competing against her for the title of Princess of the Fall Formal, Sunset employs various ways to hinder and humiliate Twilight, such as having Snips and Snails record embarrassing videos of her struggling to get used to the human world and releasing the videos online, and doctoring photos to frame Twilight for ruining the dance decorations. But Twilight's friends help to counter these ploys by improving Twilight's reputation and Flash Sentry, Sunset's ex-boyfriend, helps prove Twilight's innocence.

At the Fall Formal, when Twilight is named Princess and given the crown, Sunset has Snips and Snails kidnapSpike and threatens to destroy the portal to Equestria unless Twilight gives her the crown. Twilight refuses to give the crown up, stating that her being stuck in the human world would be worth stopping Sunset. As Twilight's friends commend her bravery and sacrifice, Sunset snaps and attacks her. Despite Twilight and her friends' efforts, Sunset succeeds in getting the crown and puts it on, transforming into a demonic creature. With her newfound powers, Sunset transforms Snips and Snails into similar-looking minions and brainwashes the other students at the school. She reveals that she never intended to destroy the portal and plans to invade and conquer Equestria with her army of teenagers. Twilight is able to evoke the powers of the Elements of Harmony when her friends move to protect her from Sunset Shimmer's attack. Temporarily given pony-like characteristics, they use the magic of friendship to defeat Sunset Shimmer and return Snips, Snails, and the rest of the students to normal.

As everyone recovers, a beaten and repentant Sunset is reduced to tears and apologizes to everyone she's hurt. She admits to not knowing anything about friendship, to which Twilight says her friends can teach her. Sunset, Snips, and Snails are later tasked by Vice Principal Luna with rebuilding the part of school they had destroyed during their attack.

Sunset Shimmer now lives with Celestia and Luna, and is dating Rimshot.

Personality
At first, Sunset Shimmer is the polar opposite of Twilight; cruel, dishonest, arrogant, mean-spirited, cold-hearted, tyrannous, evil, selfish, power-hungry, and a bully.

She uses her severe nature to intimidate others, which causes her to become Princess of the Fall Formal. She has a tendency to cruelly demoralize those who get in her way, such as when she posted humiliating videos of her online and framing Twilight for vandalizing the decorations to Princess of the Fall Formal.

Although, she had a limit to her villainous acts as she lets Spike go unharmed when her henchmen Snips and Snails captured him (as she says it "Oh, Twilight, I am not a monster"). When she puts on the crown and becomes a demon, she becomes far more sadistic and malevolent, as she hypnotizes the students and student body to become her slaves and almost kills Twilight. When she reverts back to normal, Sunset Shimmer had a total change of heart, as she has became regretful of her actions, and horrified of what she had become. Twilight asks her friends to become friends with Sunset Shimmer, as she never had a friend before. She also accepted her punishment near the end of the film to repair the school from all of the damage she made.

Sunset Shimmer is now a very nice person with many friends, and even a boyfriend!