Dipper and Mabel were barely five years old playing in the backyard when ...IT happened. Time stopped and an entity with a figure impossible to describe appeared. The being became interested in that pair of children by the Prophecy of the Zodiac. He knew that Mabel (although not a bad person) would nevertheless end up hurting people for selfish reasons, but he didn't care about that to the entity. All she was going to do would be out of curiosity and for fun.
The reason for this selfishness that Mabel was (almost) Dipper's fault- not because he was stupid but because he loved his sister so much he was willing to sacrifice his own happiness for her sake. For it so that others may maintain happiness (something that the Entity did not fully understand).
Which although was a personality worthy of the element of kindness...will sadly end with Mabel using her brother as a 'crutch' to help her through life's misfortunes...which was not a healthy relationship at all.
With the fate of many on the hands of these two, the entity decided he had no choice but to separate them in order to allow them to grow their own 'healthy', independent lives (or... at least as healthy as the situation could allow), The twins brothers personalities almost always ended up being very opposite to make themselves stand out, that's why I was going to separate them, the only problem with separating them is that these twins were needed to avoid becoming bad people because every time one of the brothers was starting to make bad decisions and the other made him understand and reconsider, but the being already had ready who was going to replace Mabel.
But while he would take Mabel away...it was imperative that Dipper continued on his current path of exploration, study and mystery solving to save his world.
But although taking away Mabel's constant interruptions would- in theory -allow this to blossom...the fact was that even with his memory erased, his heart might still ache without the love and companionship of his sister and thus derail his destiny.
No, it was too big of a risk...fortunately, the entity had a plan to kill two birds with one stone...
For there was another 'person'- in the same dimension he was planning on let Mabel no less -who had a great destiny in store for her! Sadly it would also be hindered by the well-meaning beings of that world. There love of friendship and harmony was to be admired...but their desire to put others happiness above their own was- in many way -just as toxic as Dipper and Mabel's. Worse, their love of harmony unintentionally killed much creativity which would lead to her making many avoidable and quite stupid mistakes.
He had other plans to fix that..but it was not yet time for that. For the moment he'd just focus on Twilight and Dipper...
"Let's see how you both would act if you had a people by your side who encouraged and nurtured your ideals." Said the being as he replaced Mabel with a Twilight Sparkle filly and placed it by Dipper while erasing all memory of Mabel from this world.
Meanwhile he erased all Mabel's memories - save that her name was Mabel that she had and a brother - and then he transformed her into Pegasus. The entity then left her in Equestria while erasing all memories of Twilight .
After doing this, he decided to continue bringing beings from Equestria to this world.
With his scheme set in motion, he then left and began to advance time.
"Where am I? What am I doing here? Where is my family?" Dipper was stunned at the sight of the cute unicorn filly. His heart seemed to ache at some unknown loss...but he supressed that to comfort the sobbing purple unicorn.
"Hey don't cry...I'll help you, okay?" Dipper said, wiping away her tears and giving her a hug
Twilight who was crying, was initially frightend by the strange creature... nut his comforting words and warmth of his hug helped clam and reassure her. Seeing that she was now calm, Dipper picks her up and takes her back to his house.
...
Dipper and Twilight quickly realized that no one else could see or hear her. Dipper's parents quickly believed that she was an 'imaginary friend', they then thought it was a ghost, then a demon , then- thanks to Dipper's reassurance -they were sure it was a creature from another dimension....which still terrified them to be honest...
...
The teachers and students realized that Dipper was very intelligent and a good person.
While great for his grades and future prospects, this made him a target for bullying...
Or at least it would if not for Twilight taking advantage of both her 'invisibility' and magic to protect him.
Although great for Dipper, this lead to much panic and confusion (the panicked faculty even called an exorcist at one point!).
Thankfully Twilight finally began to communicate with notes to others. Dipper's parents and other acquaintances began to be less afraid of her.
One would think that the presence of a UNICORN would cause quite the stir.
But the entity had foreseen this and had already made sure that none of those who knew of her existence would be able to pass this info to anyone who could cause a scandal.
As people got used to her, Twilight was able to convince others to test her as well. Not only did she pass the Turing Test with flying colors but she Proved she was just as smart as Dipper... well, in her own way at least...
...
As the years went by, the two tried to investigate anything about Equestria, unicorn myths or anything that could return her home... but alas, they found nothing.
At least...Until a certain new program appeared....
"My Little Pony; Friendship is Magic?" Twilight and Dipper wondered in confusion as they watched a new TV show.
"Twilight, it's you!" Dipper exclaimed in surprise.
And so the observation began. Twilight was happy to know that her existence brought so much joy to others. She also enjoyed knowing what her life would've been like by watching the adventures of her alternate self adventures as the 'Mane 6'.
She was also able to finally pinpoint WHEN she'd come to this world... some months after she'd hatched Spike. It didn't help her situation, but at least it was SOMETHING.
Of course...she would soon have problems that neither of them would've ever thought possible.
...
The day after the show, most of the students - and even some teachers - wanted to be friends with Twilight!
Twilight quickly became the most popular person in school, some were trying to be friends with Dipper just to be friends with Twilight.
Dipper was annoyed by how 'fickle' the lot of them were. switching from ostracization and fear to inclusion and 'friendship' at the drop of a hat!
Worse, Twilight discovered that she was afraid of being the center of attention of a large group of people... it helped that no one could see her... but it was starting to overwhelm her... it was getting to the point she couldn't even READ in peace!
It wasn't all bad news, study the magic in the show allowed Twilight to progress further in her studies.
Dipper was able to help as well, for although Twilight tested higher when it came to absorbing knowledge faster and retaining said knowledge....
But Dipper tested better at actually APPLYING that knowledge to real world situations and making snap decisions in fast-paced, time-sensitive and downright difficult situations(the last bit Twilight was particularly terrible with).
Thus the two complimented each other quite nicely, especially after seeing how many threats she'd be facing in Equestria. Dipper INSISTED she learn to defend herself. both with her body and her magic.
Twilight was reluctant- like most equestiran ponies, she was geared more toward 'flight' then fight -but she eventually conceded to Dipper logic and concern...
In addition to setting up a shooting gallery for her magic and dummies for her to buck they also continued to observe the show. Taking not and analyzing all of her alternate selves mistakes and how best to avoid them... like obsessing a lot over lists(realizing he had the same problem, Dipper and Twilight would go 'cold turkey' together...)
'Fans' would not help matters by continuing to annoy them, especially with demands of being turned into ponies...
Although getting more and more irritated... this did give Twilight some ideas for future research...
Dipper parents saw that they were getting more and more unhappy here in Piedmont... and began to make plans...
...
In any case, this discovery did not bring any real clues to allow twilight to return home.
Which... Twilight was more and more okay with if she was being honest.
True, Twilight was a little sad that she would never meet those friends she COULD have had... but as time passed, she realized that she cared less and less,her old home seeming more and more like a dream then anything... for Dipper had become her best friend... as well as... well... honestly, she didn't really know either... but it was something MORE...
Eventually, Dipper's parents set up a summer 'vacation' for the two, sending them to Dipper's great-uncle Stan in a strange place called Gravity Falls.
...
Dipper was pretty neutral when it came to going to Gravity Falls. He mostly went along with this for Twilight's sake, she needed a vacation BADLY... in any case as long as he had Twilight by his side... he could be happy anywhere.
He smiled as he watched Twilight almost screaming and jumping with excitement. A new place to explore was always fun!
They were both happy while traveling to Gravity Falls... neither noticed that the bus driver would sometimes look at them and rub his eyes in disbelief.
...
Stan Pines was waiting for his nephew at the bus stop.
Although excited to spend the summer with family... He alsoknew he had his work cut out with the kid to turn him into a real man, the poor sap apparently still had an imaginary friend for crying out loud!
Stan didn't realize that his cheapening out with the phone lines had distorted many things that Dipper's parents had tried to tell him...
...
As soon as Twilight and Dipper got off-
BOOM!
Stan used the smoke bomb to appear out of nowhere-
ZAP!
Twilight- thinking they were trying to attack Dipper -struck back at a surprised Stan. Knocking him flat on his back
"Whoa, I must've drunk down more of my stash tehn I thought... did anyone get the number of that purple unicorn?!" Grunkle Stan grumbled in a daze as he helped himself back up.
Before either could respond, Dipper and Twilight realized that everyone was looking at them... more specifically, at TWILIGHT.
"Twilight...! can they... see you?" Asked Dipper shocked.
"It can't be! Is that a unicorn?!", shouted someone.
"She is so adorable!" said someone else.
"I... I think they can, Dipper." Twilight said stunned. "Wait, how the hell can they see me?"
Dipper and Twilight did not have time to process this information as Stan- always quick to detect an opportunity to earn money -quickly yelled:
"PHOTO WITH THE PRETTY UNICORN 25-! No, 100$! Take advantage of this deal while it lasts!"
Twilight, still stunned... goes along with it... what else can she do?
"Get your Unicorn Merchandise!" Stan quickly turns to Dipper, "Boy! I've got some purple paint in the gift shop drawer! Tell Soos to drag out the corn-a-corn crap! Chop, Chop Boy! Time is money!" He whispers to Dipper as he hastily pushes him along.
Dipper- equally confused -also rolls with it But, ONLY after making sure that Twilight is okay... not knowing what else to do... they decide to solve this later when less people are about.
As the Mystery Shack's work hours were ending, a red-haired teenage girl inside the cabin spotted Twilight.
"Twilight? No way!" She quickly started making some calls.