Depiction In Friendship Is Magic
As the leader of this village, Starlight has the philosophy that "true friendship" comes from being equal: no more or less talented than another and that having special talents of any kind only leads to disharmony. To this end, Starlight uses a magical artifact called the Staff of Sameness to remove her followers' cutie marks and replace them with equal signs.
When Twilight Sparkle and her friends arrive at the village, Glimmer sees this as an opportunity to spread her message of equality to all of Equestria. She leads the Mane Six to a vault of cutie marks outside the village, where she takes their cutie marks from them with the Staff of Sameness and tries to convert them over to her way of thinking. Fluttershy, the only one among Twilight's friends who appears to convert, spies on Starlight and discovers that her own equal sign cutie mark is actually makeup.
Using a bucket filled with water, Fluttershy and her friends expose Starlight's secret to her followers. It is also revealed that the Staff of Sameness's ability to remove cutie marks is Starlight's own magic and that the staff is an ordinary piece of wood. She attempts to leave with the Mane 6's cutie marks but is thwarted by her former followers. Having lost her followers' faith, Starlight retreats from the village and escapes to parts unknown.
Glimmer makes her second major appearance in The Cutie Re-Mark (Parts 1 and 2) as the main antagonist, similar to Sunset Shimmer in the first Equestria Girls film. She uses a time spell created by Star Swirl the Bearded to travel back in time to the day Rainbow Dash performed her first sonic rainboom. When she prevents Rainbow from performing the rainboom, she changes Equestria's history, and she takes steps to ensure that Twilight and Spike fail to stop her from doing it as well.
As Starlight and Twilight continually clash, Starlight creates numerous alternate futures in which Equestria falls into ruin. Twilight brings Starlight with her to one future in order to show her the consequences of her actions, but Starlight refuses to accept the truth. As Twilight cannot understand why Starlight acts this way, Starlight reveals that when she was a filly, her best friend Sunburst earned his cutie mark and left for Canterlot, leaving a young Starlight without a friend and with a fear of losing potential future friends. After bargaining with Twilight, Starlight decides to allow the rainboom to occur, restoring the present day.
Realizing the error of her ways, Starlight accepts whatever punishment that Twilight and her friends give her. Instead of punishing her, however, Twilight and her friends decide to teach Starlight about the magic of friendship, and Starlight feels reinvigorated with new life.
In The Crystalling (parts 1 and 2), Starlight has taken up residence in the Castle of Friendship as Twilight Sparkle's student, and Twilight presents her with her first friendship lesson: reuniting with her old friend Sunburst. Because of her past actions, Starlight is very nervous about seeing Sunburst again. When the two finally meet again, their dialogue is mostly awkward stuttering.
After a failed reunion, Starlight is convinced that Sunburst no longer wants to be her friend. However, when she approaches him later and discovers that he did not become an important wizard like she believed, Starlight learns that they both have their own histories of shame. The two then help Twilight and her friends save the Crystal Empire from an eternal winter. With their friendship rekindled, Starlight and Sunburst say goodbye on much friendlier terms and promise to keep in touch.
In No Second Prances, after several attempts to make friends in Ponyville end in failure, Starlight meets and befriends with Trixie Lulamoon. When Twilight expresses skepticism in their relationship, Starlight feels that Twilight doesn't trust her to make her own friends. However, when Trixie admits that she became friends with Starlight partly to one-up Twilight, Starlight is emotionally hurt. With Twilight's help, Starlight makes amends with Trixie and helps her in her magic show.
In A Hearth's Warming Tail, Starlight is unenthusiastic about celebrating Hearth's Warming Eve and considers it no different from any other day. However, after Twilight reads her the story "A Hearth's Warming Tale", Starlight gains a new appreciation for the holiday.
In Spice Up Your Life, Glimmer briefly appears helping Twilight to reactivate the Cutie Map after the events of The Cutie Re-Mark caused it to go inactive.
In The Times They Are A Changeling, Starlight accompanies Twilight and Spike on their visit to the Crystal Empire to visit Sunburst and Flurry Heart. She is present when Spike's changeling friend Thorax exposes himself by accident, and she is quick to help defend Flurry Heart against the suspected changeling spy. Later, when Spike pleads with his friends that Thorax deserves another chance, his words strike a chord with Starlight, who comes to accept Thorax as a friend.
In Every Little Thing She Does, Starlight excels at her magic studies, but she avoids her friendship lessons. To fix this, she takes on several friendship problems at once with Twilight's friends, in order to impress her. In an effort to make things easier, she uses the Fiducia Compelus spell to mind-control the ponies into following her orders. Unfortunately, the ponies follow her orders so exactly and literally that things quickly spiral out of Starlight's control. After Twilight helps fix her mess, Starlight confesses that she avoids her friendship problems out of fear of failing and being a disappointment to Twilight. With Twilight and Spike's guidance, Starlight apologizes to her friends for using magic on them, and she learns to address her friendship lessons more naturally.
Depiction In Equestria Girls
In My Little Pony: Equestria Girls – Mirror Magic, when Sunset Shimmer returns to Equestria, Starlight gives her a new magic journal and becomes very interested in the world on the other side of the Crystal Mirror. She convinces Sunset to take her along, and she transforms into a human upon her arrival.
When Sunset and her friends become trapped in limbo through Juniper Montage's enchanted mirror and Juniper starts terrorizing mall patrons, Starlight convinces her to stop by offering to be her friend. In the end, Starlight helps Sunset to not worry so much about future dangers and to live in the moment, and through a message from Twilight in Sunset's journal, she is allowed to stay in the human world for a few more days.
Glimmer returns in My Little Pony: Equestria Girls and Sofia The First – Crystal Prep High Rocks! as Sebastian tells her, Sunset and Twilight that Crystal Prep Academy's Shadowbolts team gets a new member. He also mentions that the new member is Sofia, the kid from Enchancia who left the old Disney Junior TV series Sofia The First, much to Rainbow Dash's surprise. Rarity mentioned that Sofia The First is Sweetie Belle's favorite Disney Junior TV shows of all time and Sebastian decided to travel to Salzburg, Austria with the Cutie Mark Crusaders. Sebastian and Gordon decided to borrow Sunset's Kia Telluride as she follows Applejack in her Volvo XC90, much to the CMC's confusion. He also announces that Crystal Prep's Shadowbolts team added Sofia as their new member, much to Sweetie Belle's surprise. In addition, she drives the same Mercedes-Benz C-Class just like Apple Bloom and Silver Spoon, although Apple Bloom's car is a Mercedes-AMG C63 S Sedan.