Sunset Shimmer (voiced by Rebecca Shoichet) used to be the main antagonist in My Little Pony: Equestria Girls and now she's one of the main protagonists in My Little Pony: Equestria Girls - Rainbow Rocks.
Throughout Equestria Girls, Sunset Shimmer is shown to be mean, short-tempered, greedy, bratty, 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 Sparkle'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. Despite this, 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 fierce and goes so far as attempting to dispose of 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 Equestraia Girls - Rainbow Rocks, Sunset Shimmer appears to have completely changed from her old self and started believing in friendship, becoming friends with her former enemies, even Twilight Sparkle, 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.
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.