Sunset Shimmer appears in the film My Little Pony Equestria Girls as the main antagonist. She is a 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. Later, she puts on the crown and transforms into a demonic-looking winged humanoid. Once she is defeated by the main characters, she returns to her human form and apologizes for her actions. Afterward, Vice-Principal Luna hands her a masonry trowel.
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.
In Rainbow Rocks, Sunset Shimmer is heavily ostricised by her schoolmates for what she did at the Fall Formal. However shea has also become humble and loyal. In the kitchen scene, she provides support for Twilight when she is feeling discouraged.