Twilight Sparkle

Physical
Lavender-purple unicorn pony with straight well-kept navy blue mane and tail, with two individual stripes, one pink and the other purple. She has dark purple almond-shaped eyes.

Her cutie mark is a six-pointed star, with small white stars surrounding it, symbolising her vast magical abilities.

Biographical
Both an antagonist and protagonist to the story, the curious case of Twilight Sparkle served as the main source of tension throughout the first book, and most of the sequel.

Quick to think of plans and smart enough to carry them out, Twilight is renowned for her intelligence and wit. Stubborn as a mule, Twilight is blatantly opinionated and believes her way, is usually the right way.

Led to believe humans are evil, she developed a strong feeling of hatred and fear towards them. Throughout the story, she also grows hateful towards her own friends, thus making the group wary of her health.

Backstory
As a very introverted filly, Twilight grew up hardly speaking to anyone, only her brother, Shining Armor, whom encouraged her to socialise more.

To no avail, Twilight remained a social outcast as her only interests were learning and studying for academic events, desperate to become the best in her field.

After Rainbow Dash's sonic rainboom triggered her magical outburst, hatching Spike's egg and bringing chaos to the Entrance Exam, Celestia saw her magical potential and took her on as a personal protégé. She went on to study magic in its highest fields, and even made some friendly acquaintances at the School for Gifted Unicorns.

After years of studying, nearing the foretold return of Nightmare Moon, Celestia set her upon a path to encounter and befriend the other ponies that would and later represent the elements of harmony, triggering the Revival of the Elements. Twilight was sent to Ponyville to learn about the magic of friendship, where her social interaction skills developed.

The events over MLP:FiM's first 3 seasons then occur.

Twilight's Mission
Upon Nah'Lek's return and the Titan's Orb being sent to Earth, Princess Celestia had no choice but to send a company of ponies after it to retrieve the shards. She needed ponies with adept abilities, and the capacity to work well together and not break under the odds.

Who better than the six representatives of the Elements of Harmony?

Selecting the six to embark upon this question, Twilight was appointed the leader, her analytical skills, magical capabilities, and strategic mindset was the ideal candidate to lead the group on such a mission.

Celestia's Vision
Summoned to Canterlot to receive the mission briefing alongside her friends, Twilight was privately taken aside by Princess Celestia; it was previously assumed Twilight had merely been given an additional pep-talk, but it was later revealed that the Princess had induced her into a vision of the future, wherein Ponyville was ransacked by Orcs, which Twilight misinterpreted as humans.

This vision sparked her fear and hatred for humans, terrorised by the thought of her home being destroyed by this alien race, hellbent on destruction and death.

As the vision concluded, Celestia warned Twilight that the scenario would come true, should Callum return to Equestria with the company upon the relocation of the Orb shards. Twilight vowed he'd never set foot on Equestrian soil, and agreed to not befriend him.

Callum and Twilight
When the group first arrived to Earth, Twilight deployed a cloaked camp behind a shed in Callum's large garden, wishing to study the boy first before engaging with him. Observing him for two weeks, Twilight quickly learned of Callum's broad persona, he was intelligent, he was strategic, he was physically strong, and he was gifted in combat.

While these attributes explained why the group had been sent to him, Twilight very quickly feared such attributes in a human, after witnessing the future at Ponyville, she believed Callum would lead to Equestria's downfall if he was met with too much trust and friendliness.

On the night Callum discovered the ponies and made first interaction, Twilight was instantly standoffish, taking Celestia's advice too literally. She was very quick to reveal an unrest within her, which Callum responded with understanding and kindness, although Twilight quickly perceived his approach as a deceitful trick, and was only driven further into her paranoia.

Throughout the story, Callum and Twilight constantly butt heads and drift further and further apart, the tension between them grows dangerously volatile, and only worsens as Twilight commits various atrocities towards Callum, hoping to bastardise his image by aggravating him to the point of violence.

This backfires drastically, and only besmirches her own image, as she's eventually caught in committing these atrocities. Due to the Fel's corruption, the young unicorn is deluded into believing all of these situations are a connected conspiracy, designed by Callum in an effort to destroy her.

Losing Dominance
Over the quest, Twilight grows more and more unstable, and starts to push her friends away, under deluded judgement, she became obsessed with Callum, desperately trying to depict him as a monster to the others. As Callum proved her wrong time and time again, she began to lose her grip over the group, as they trusted him and his guidance over her.

This only furthered her hatred for him, convincing her that he was manipulating the group against her, and plotting to overthrow her and have her killed.

The Fel
Anti-mana, better known as the Fel, the most vile and ungodly substance in the universe.

Upon absorbing some left over residue from the Titan's Orb, Twilight was unknowingly exposed to a minuscule amount of this sickly material, and it ever so slowly took control of her mind, altering her brain chemistry and enhancing her magical abilities. Feeding off her incredibly large mana well, the Fel slowly twisted her judgement and moral compass, slowly corrupting her and driving her mad, to the point of insanity.

Twilight's fear and hatred for the human race only fed the Fel further, forcing her to believe Callum was truly out to kill her, and lead an army of men to Equestria to destroy it, like she'd seen in Celestia's Vision.

As the Fel slowly replaced her mana, her capabilities grew much stronger, her magic doubled in strength and her body grew more resilient, able to heal remarkably fast. By the time she became fully corrupted, she was as physically strong as Applejack, and magically strong as Celestia herself; however, she'd also completely lost her mind by this point; upon her clifftop fight with Callum, she was consumed by blood-lust, and turned on her own friends after believing she'd finally killed the human she'd grown to hate.

Twilight's corruption was eventually brought to an end with the assistance of Hawnu Rey'eng, whom provided magical collars that allowed Callum to enter Twilight's brain, where he located Twilight's corrupted mana well, and drained it, starving the Fel to death, sacrificing Twilight's use of magic in the process.

With the Fel gone, Twilight remained mentally scarred from it's damage, causing her to have rare outbursts of aggression, such a corruption would take years to fully recover.

Nah'Lek's Shade
Additionally to being corrupted by the Fel, her mind was invaded by Nah'Lek the Defiler, using the Fel as a gateway to her mind. Once inside, Nah'Lek began tormenting Twilight in her sleep via an endless stream of nightmares, hoping to manipulate her into suicide. As Twilight showed resilience, Nah'Lek's torture grew more and more brutal, until he eventually resorted to flaying her alive, and later even sexually assaulting her via extremely painful methods.

Upon learning of Callum's assistance to recover the Orb shards, Nah'Lek chose to keep her alive, by promising her an end to the nightmares, should she kill Callum. Twilight fought against this urge for the entire duration of her corruption, desperately clinging onto her sanity and refusal to disappoint Princess Celestia. She refused to kill Callum to the best of her ability, only attempting to at the pinnacle of her bewitchment when Callum attacked her, after purposefully letting Bunnie die.

When Callum entered Twilight's brain, Nah'Lek's Shade sought after him, trying to hunt him down before he could cure her. While unable to locate Callum in time, he soon found him in the mana well, and confronted him, only to be taken off guard by Hawnu Rey'eng, whom fought the Defiler in the cavern, before overpowering the guardian and making a final haunting attack on Callum, tearing his spine out through his chest.

With Twilight cured and Nah'Lek outsmarted, he hijacked the collars to enter Callum's mind, to conduct further tortures on a new victim...