Starlight Glimmer is an adept, Eastern unicorn that is Princess Twilight Sparkle's pupil. Under her guidance, Starlight hopes to both learn friendship and atone for her past actions.
Background
Fillyhood
Since she was young, Starlight has always been proficient at magic. Aided by her best friend, Sunburst, she learned at an exponential rate, eventually being able to do spells such as cleaning an entire kitchen.
With Sunburst earning his cutie mark, he was shipped off to Celestia's School for Gifted Unicorns. Bitter from his departure, Starlight continued on with her magic studies without any sort of restraint or guidance. She now had a goal: remove cutie marks from Equestria.
The Cutie Map/Remark
In a remote location in Equestria, Starlight had deployed her scheme for "equality"- remove a pony's cutie mark, teach them conformity, and brainwash them into the mentality of friendship is rooted in equality.
Upon the Mane 6 arriving, Starlight greeted them with hopes of persuading them that her way is the true way to friendship. When this failed, Starlight lead them to the Cutie Mark vault, where she sprang her trap and removed their cutie marks. She was ultimately thwarted through Fluttershy's efforts of acting converted, through which it is revealed Starlight retained her cutie mark.
Though her villagers rebelled, Starlight attempted to escape with the Mane 6's cutie marks, only to be defeated through the collective efforts of all. She retreated, only to follow Twilight through her adventures(throughout Season 5).
Her following ceases upon Twilight giving a lesson at the university, where Starlight reveals herself, to which Twilight assumes is just a nervous reaction. Upon she and Spike's arrival to the Castle of Friendship, they find Starlight laying in wait by the map. Starlight then casts a time manipulation spell from one of Starswirl's old spells, which she later boasts about modifying to suit her needs. As Twilight and Spike follow, they're whisked to the start of it all: the race. As Rainbow Dash is about to perform her rainboom, Starlight places her into suspended animation, severing the "cutie mark connection" between the Mane 6 and sending Twilight back to the "present."
On Twilight's return, Starlight get the jump on her, revels her plans, and proceeds to prevent the moment with each attempt. After several duels, the two finally stalemate, with Starlight agreeing to be "game" for an eternal fight. In an act of desperation, Twilight grabs onto Starlight, sending the three into the worst possible outcome. It's here where Starlight reveals her past, including her reasoning for the hatred of cutie marks. After returning to the past, Starlight decides to concede and allow everything to happen. With their return to the current timeline, Starlight is eventually taken in as Twilight's only pupil.
Personality
Starlight, despite her previous guise, is a rather timid mare that dislikes uncertainty and finds it difficult to relate with ponies. While she does enjoy talking to and attempting to befriend others, Starlight is prone to having outbursts of nervousness and acting slightly awkward whilst doing so. When interacting, Starlight tends to be very blunt, to the point, and, at times, challenging. She also tends to be manipulative when it comes to getting what she wants, be it through use of magic or persuasion.
In addition to learning about friendship, Starlight is also tasked to help the Princess with spells and other various tasks as needed. With each of her lessons, she takes pride in her successes and isn't afraid to show it.
When not busy with her studies, Starlight enjoys doing magic research of her own, going out for trots through town, and having the occasional spa day.