Mabel was Dipper's twin.
Simple enough, right?
Wrong.
Dipper had always been somewhat of an unconventional child. He loved nothing more than to curl up with a good book while other boys his age were out playing sports; he loved mysteries where others liked action, he loved planning and lists while others procrastinated. Dipper was blessed, or cursed, with an overactive mind. While he caught on to ideas twice as fast as children his age, his ever churning brain gave rise to insomnia, and he tended to fixate on things that were beyond his control.
Mabel was the opposite. Her mind flitted from subject to subject faster than anyone else could seem to comprehend; she was attracted to shiny things with magpie-ish intensity, and she was as silly as her Dipper was serious. Where Dipper was reserved, Mabel raced ahead; where Dipper was timid, she was bold. Things that were beyond her scope were brushed aside as unimportant, and the world to her was a friendly and colorful place.
They were total opposites, but they balanced one another perfectly.
Though Mabel didn't let on, she was as sensitive as Dipper; after a group of children at school one day had laughed at her skinny arms, she had begun wearing sweaters, and she hadn't stopped even in the summer. Dipper still remembered her face as the others had all pointed and jeered; he had vowed with all the fury a preschooler could muster that no one would ever make Mabel cry again.
From that day on, protecting his sister Mabel had become Dipper's mission. On the rare occasion she got in trouble at home, Dipper would stand up to his parents and invariably get himself grounded as well. Bullies at school were dealt with quietly and savagely at recess, and the bemused comment by Mabel at the dinner table one night about how everyone at school was so nice all the sudden had been well worth the bruises. The teachers were unsure as to how to deal with the strange twins, but it was an unsaid law among the administration that the two were to always be in the same class. Though they would socialize with other students in the room, every few minutes the two would look around reflexively for one another, checking that the other twin was present and safe. If one of them fell on the playground, the other would be right there in the nurse's office with them, watching the adult apply the Band-Aid with wary eyes. If one of them was sick, the other would tell stories and bring soup until the invalid was better. If one of them had a nightmare, the other would crawl into their bed and offer comfort.
As the years went on that protective instinct became much more pronounced in Dipper, and he constantly watched over his sister, doing his best to keep her out of trouble. Occasionally her silliness and lack of attention could get her injured, and he constantly watched for things that could trip her as she rain around aimlessly. He tried to keep her from eating too much sugar, and acquired a first aid kit for the times when she would run into something on her rambles. His parents had been amused by their son's seriousness, and Dipper knew that they were severely underestimating just how far he would go to keep Mabel safe.
Then came Gravity Falls. The town was a mess, full of crazy characters and secrets; an irresistible place for an active mind such as Dipper's. Grunkle Stan's Mystery Shack became more of a home than where his parents were, and the woods around the building housed unimaginable creatures and secrets. With these secrets, however, came danger. Dipper did his best to keep her safe on their adventures, pulling her out of harm's way on several occasions, and always making sure to place himself in front of Mabel if they were being threatened.
There were bullies and idiots in every town, and Dipper found his two quickly: Gideon Gleeful and Pacifica Northwest. Gideon had become his arch-nemesis as soon as he'd begun sniffing around Mabel; Dipper was extremely choosy about the boys he would accept around his sister, and that white-haired punk didn't fit the criteria at all. The Gnomes were bad enough, and he'd easily gotten rid of them with his sister's help, but Gideon was relentless. What was worse, Mabel didn't like him either, yet he still wouldn't take no for an answer! Punching him had felt amazing, though Dipper would have preferred that the battleground wasn't in a giant robot with his sister's life at stake. Watching her be ripped away from him by that huge, metallic fist had been awful, and he'd never been so furious—as soon as the creep got out of jail Dipper would make sure to punch him again, just for good measure.
Pacifica Northwest was a different matter entirely; she was just a bully, plain and simple. Dipper knew that when they started middle school, which was alarmingly soon, there would be more like her—ready to pounce on Mabel and tear her apart bit by bit. He considered Pacifica to be practice. After revealing that her ancestor was in fact the village idiot and not the founder of Gravity Falls, Dipper doubted that she would cause any more trouble. If she did then he would be there, ready to remind her of that poop shoveling great great great grandpa.
Mabel was Dipper's twin, but she was also much more than that. She was his best friend, the one he told all of his secrets to, the only person who had ever seen him cry. Mabel was always there to cheer him up when he was down, she gave him confidence when he had none. Mabel was his whole world, and though there were others in it like Grunkle Stan, Soos, and Wendy, Dipper knew that he would hold her above them no matter what. Without Mabel, there wasn't a Dipper. He planned to protect her as long as he physically could, through everything and for always, no matter the cost.