Sunset Shimmer is a student at Canterlot High and is an adopted daughter of the Wubb Girlz. She is also the lead vocalist/guitar player of her own band, the Rainbooms.
Sunset Shimmer used to be a Unicorn pony living in the world of Equestria. There, she was originally the student of Princess Celestia. But she had a troubled childhood; when she was born, her parents specifically said they "never wanted a kid", going so far as to call her a "waste of time and space" and abandoned her in the streets to die. School life for her wasn't any better. Throughout her time as Princess Celestia's student, she was bullied by many of the colts and fillies. There were even some who "befriended" her, but ended up betraying her in the end. That was where it all started; since then, she had been very bitter and cold to whoever she met; even after leaving Equestria and coming into the human realm, she maintained that persona for a while.
All seemed lost for Sunset, that is until she was adopted by Ariel Phantom Lotiz, the wife of the Wubb Girlz' lead singer, Shine. The three girls, plus Shine's younger sister, Glitter Ortiz, got on great; however, Shine's other sisters, Sparkle and Shimmer were highly skeptical of her. That came to a head during the fiasco with Twilight Sparkle, the newly-crowned "Princess of Friendship" coming into the human realm to retrieve her crown, which had been stolen by Sunset.
During the final showdown with her, Ariel tried to reason with her now-demonized daughter, but Sunset unwittingly injured her, which caused her to have a brief sense of remorse, giving Twilight and the human counterparts of her Ponyville friends the chance to defeat her. After her defeat, Sunset broke down in tears because of what she did, admitting that she "had no knowledge of friendship" and because of her actions, all they did was "drive people apart". Twilight reminded her that she had the other 5 girls, plus Shine and Ariel to help her out.
Ever since then, Sunset has been trying her utmost to show the world that there's another side to her. And she was able to have her chance when The Dazzlings came to town. By this time, her friends had formed a band called "The Rainbooms", but they unintentionally left her out. But when she saw that something off was going on at C.H.S. after Adagio, Aria and Sonata came into the picture, she was the only one who'd kept a level head and saw through the Sirens' ruse. But over the course of the film, as the band kept arguing, Sunset was also haunted by memories of what she did in the past, which the other students weren't afraid to keep bringing up. Her shining moment came during the face-off with the Dazzlings; she saw that her friends needed her, and after a moment, she joined them and aided them in bringing down the Sirens once and for all. Since then, she's garnered a new respect from the students at Canterlot High.
Sunset originally was a bitter individual, believing, as she'd once put it, "ambition is what I believed to be the only way to set me free", meaning that because of her hard past, she felt that she needed to not show any "soft" qualities and be hard-headed and cold to anyone who stood in her way. But when she'd met Ariel and Shine, they helped break down her walls somewhat, but that didn't sink into Sunset until after her defeat by Twilight and the others. But ever since then, she's shown a kinder side to her as she strives to show the world the kindness she should have. She's also shown a forgiving and compassionate side, especially after the Anon-A-Miss scandal. On top of that, she's also shown a high level of intelligence, especially by performing scientific experiments on her friends (much to her mother's amusement).
One winter, Sunset had revealed that for the longest time, she'd been spending Christmas alone (prior to her adoption by Ariel and Shine, of course). Her friends decided that they should help her out and spread more Christmas cheer. They'd planned a series of slumber parties at each other's homes, leading up to them all staying at Applejack's farm on Christmas Eve. Sunset was thrilled about it, having never really been involved with such activities. But during the following weeks, some students noticed a website made by someone called "Anon-A-Miss", whose page and color choices looked eerily like that of Sunset. At first, her friends didn't think anything of it, until one day when Applejack showed Sunset that more embarrassing photographs and secrets about the girls were put up on the page. Sunset tried to declare her innocence, but the others broke off their friendship with her, leaving her behind in tears.
After school that day, Ariel found Sunset huddled up on the couch crying bitterly. After talking with her daughter and trying to calm her down, she went and talked with Sunset's (then) ex-friends, which blew up in her face. They'd claimed that Ariel was supporting "Sunset's" behavior and habit to expose embarrassing secrets about them. The hatred went on for days, until finally, Sunset was able to gather the evidence she needed to clear her name. It was revealed that she had been framed by Applebloom, Scootaloo and Sweetie Belle, who were jealous of their big sisters spending more time with Sunset and less time with them. Despite all that, Sunset and the others were able to forgive the trio, but they had to tell Principle Celestia what they'd done.