So I forced my dad to start watching GF (he loves it, yes!); he really dislikes Bill. At which I am SUPER offended - I'm one of those people who falls in love with the mean villain who had next to no like-able characteristics xD

"Mabel!" Dipper said ecstatically and jumped from his bed; he felt that he'd slept better that night that he had his entire life: perhaps a side effect of being visited by a dream demon?
"What?" she mumbled groggily, still half-asleep.
"The most amazing thing just happened! I was asleep and- "he was cut off by his twin:
"I don't care, tell me later," she slurred.
"But Mabel, it was a dream demon! He- I- he was a triangle shaped demon!"
Mabel rolled over to face him and squinted through tired eyes at her brother, "Dipper, it was just a dream, tell me about it later." She closed her eyes in an attempt to go back to sleep when she was shaken awake; groaning, she opened her eyes and sat up, giving up on her wishes to sleep the day away.
"Look, I have proof," he smiled as he showed her his hands; she read the message first and glanced at the turkey drawing.
"A turkey? Good to know you're finally getting into drawing, bro-bro!" she hopped down from the bed and started getting some clothes for the day.
"What? No, Mabel! I didn't draw it, the demon did- Bill; he said his name was Bill!"
"Why would a dream demon visit you? I mean no offence, but you aren't exactly the funnest guy in the world." Mabel said, picking up a pink, sticker-decorated hairbrush and running it through her brown bed-hair.
"Hey I am too the funnest guy in the world!" The boy argued before muttering his complaint on her lacking to use real English words; he sighed: "and I don't know why he wants me either, but he said he wants me to get him a body."
"A body; what's that supposed to mean?"
"He said something about being in the Mindscape and he wants a physical form or something."
"What's the mindscape?"
"I don't know, he said he'd tell me later."
Dipper was surprised when his sister hesitated for a moment before replying, seemingly having thought seriously about something for the first time in a long time: "I don't know, Dipper. I've seen enough scary movies by now to know that you shouldn't do anything for the devil."
"He's a demon, it's completely different. Besides, they're just movies; they're probably entirely fake, I mean, what are the chances that any of the movie directors actually ever met a demon or even the devil?" the brunet tried to persuade her.
"I guess so…"
"Besides, he offered to show me around and help me find more monsters in Gravity Falls in return, so I'm getting something, too," he explained.
The girl perked up at that; "oh so he can help us today, right?"
"Yeah, totally; he said he'd make sure we didn't get hurt, too!"
"Awesome! The mystery twins are back in action!" she exclaimed, throwing her hands in the air happily. "And now we have someone else to come with us," she gasped and her eyes shone bright with a colourful spectrum of imaginative ideas hiding behind the chocolatey orbs, "we should start a club! The mystery club! I bet Candy and Grenda would join, too!"
"Woah, Mabel," dipper chuckled and put his hands up to signal her to calm down. "It's just Bill, not an entire club. He's only helping because of the deal, remember?"
"I still say we make a club. I'd be the fabulous, cool leader who everyone loves while you could be the boring, mean leader who secretly has a soft spot for girly pop songs."
"We're not making a club."

In Mabel's imagination, she stood upon a tall cliff that hung over an outrageous ocean; the waves violently crashed against the edge of the chalky-white cliff and ice cream-pugs with banana-wings could be seen flying in the rainbow coloured sky behind her. Her now-brushed hair fanned out elegantly in the breeze and-

Dipper shut the attic's window. He shivered from the cold grasp of the wind that tugged his hair back – revealing his birthmark - before interrupting his sister's fantasies: "so…" he drawled out, "you ready to go, or..?"
"We need to see Wendy! She's back today and said she'd come visit as soon as possible!" Mabel said.
"Oh yeah," he smiled, "race you to the bathroom!" he ran before his sister could process the challenge and grabbed his tooth brush as he entered the small room. Squeezing a small amount of paste onto his brush, he rinsed it under the water briefly before going to work on his teeth; Mabel ran in and did the same, opting to sit on the edge of the bathtub as she did. "So," she drawled around her toothbrush, "you think Wendy'll go on a date with you now?" Mabel took out her toothbrush and taunted playfully, wiggling her eyebrows suggestively before plopping the brush back into her open mouth.
"Mabel!" the younger twin almost choked on his tooth brush at the sudden brutality of Mabel's straight forward question; "how many times do I need to tell you? I don't like her!"
She pulled the item from her lips to point it at her brother accusingly; pasty-white saliva dripped from the end and onto the already dirty floor while she narrowed her eyes at him. "We both know that's a lie."
"No it's not."
She hummed in thought before continuing: "then we should get you a new girlfriend! Or… boyfriend? Do you like boys or girls? Or both?"
"What? Stop asking me things like that, I don't even care!" he blushed slightly.
She laughed, "what about Bill? You could make a deal with him to be your boyfriend," she joked and laughed again.

Dipper started choking on his own saliva as she suggested it, and his face reddened either from embarrassment or lack of oxygen – which one, Mabel didn't know – and he dropped his toothbrush into the sink before turning on the tap, cupping his hands and drinking from them to clear his throat. After a minute, his face returned – mostly - to its usual pale colour and he glared at his sister who was grinning at him. "No." He stated bluntly.
"I was joking, bro-bro; lighten up!" she laughed.
"Good, I don't want you spending this summer trying to get me and Bill to start dating," Dipper rinsed his toothbrush after Mabel did and they made their ways back to the attic to fetch clothes for the day. "That'd just be weird," he mumbled.
( ͡° ͜ʖ ͡°)

The twins ran to the shambled gift shop downstairs after having finished their morning routines; smiles were plastered to their faces from the excitement of finally being able to see their red-headed friend once more. However, the large smiles shrivelled up and died in disappointment when they saw the shop empty, no one behind the counter, no Wendy and shockingly no customers. "Where is she?" Mabel asked, clearly feeling dejected at the absence of the older girl.
"I don't know, Stan said she was back today. Maybe she got delayed..?" Dipper reasoned. They turned to leave when Mabel let out a squeak next to him and then there was a sudden yet soft pressure that wet the back of his neck, causing him to also squeak. Reaching back to wipe at the damp area of his neck, he turned and saw a small, blue, plastic water-gun being aimed over the counter by someone hiding behind it. Mabel ran to the side of the counter to find the familiar smirk of an all the more familiar red-headed teenage girl.

"Wendy!" Mabel cried and tackled the girl with a vice-like hug, laughing alongside her friend.
"Hey dorks," Wendy greeted, grinning to Dipper when he stood behind his sister with a smile splitting his face in two. She was clad in the same attire she'd worn the year before, but in place of her old hat was Dipper's old pine-tree cap, dirtied and worn; despite its tattered appearance, Dipper was glad to see it in that state, it meant that his older friend had worn it a lot over the year. "Good to see you're still rocking my hat," Wendy said, standing as Mabel finally released her.
"Of course!" Dipper agreed enthusiastically.
Wendy tugged the warm, woolly hat from the boy's head, leaving him feeling as though he was stark naked before his old cap was placed in its stead and the bill was pushed down over his visage playfully. Pushing it back up, he fixed his fringe under the cap and looked at Wendy who was now donned in the woollen headpiece he remembered her for.
"You dudes ready to hang out?" She grinned and pointed to the ceiling; Dipper took it as a hint to signal that she meant the roof.
"Yeah," Mabel cried, "Roof time! Roof time! Roof time!" she chanted, soon followed by Wendy; Dipper laughed before joining them happily and following them to the roof.
( ͡° ͜ʖ ͡°)

After a long few hours of chatting with Wendy, tossing pinecones with her, and generally catching up with her, the younger teens decided that it was finally time to head out into the woods in search for some mysterious monsters or charismatic creatures.
"Alright, Bill said he'd help us out so…" Dipper said; they were standing at the edge of the thick throng of trees; the journal was safely tucked into the boy's vest and he glanced at their surroundings, trying to catch sight of the floating demon – how hard could it be to find such and obnoxiously brightly coloured triangle?
"Not too hard, Pine Tree." Came the casual reply and Dipper jumped, head spinning back around so quickly his neck could have snapped. "Got to say, kid, I think you look a heck of a lot better with that hat than the one you had yesterday."
"What- I told you my name is Dipper! Stop calling me Pine Tree!" Dipper argued, pointing accusingly at the demon, who laughed and just as Dipper was about to continue, he was cut off by his sister;
"Who are you talking to?"
The brunet stared at his sister for a moment, shocked at her question; could she not see Bill? He had mentioned about being stuck in the Mindscape, did that mean only one person could see him at a time?
"What'd I say? Smart kid; this is why I picked you and not Shooting Star," Bill pointed his cane at Mabel, who was still looking at him, confused.
"She can't see you?" Dipper asked, looking at him.
"Well, she could if I wanted her to. But I don't."
"What; why?"
"Can you see he face? That expression is priceless!" despite not having a mouth, it was clear that he was smiling from the squint of his single eye and he wiped non-existent tears from the large orb. Dipper then looked at his sister again. Her eyebrows were furrowed in concern as she looked at him intently, her typical smile flipped into a frown;
"who are you talking to?" she asked.
"Bill," he gestured to the space where said dream demon floated, "he doesn't want you to see him for some reason."
"Aw! Is he shy?" Mabel gushed, pushing her hands into her cheeks as she imagined the scene, "that's so cute! Or should I say… acute?" she giggled at her own joke.
"Cute?!" Bill repeated, clearly offended by Mabel's choice of words.
"I think you offended him." Dipper said.
"Is he blushing? I bet he's blushing. I bet he's adorable!" Mabel rocked her face in her hands as she spoke but then let out a yelp as a small stone landed on her head and she looked up to see where it had come from before looking back to Dipper and gasping at the sight. Floating next to him, spindly, black arms crossed and looking rather annoyed, was the floating, yellow triangle that her brother had spoken of. "Bill?" she asked looking at him.
"That's right, Shooting Star, now be quiet or I won't help you."
"Wait so Mabel can see you now?" Dipper interjected.
"Yup," Bill confirmed, popping the 'p' at the end of the word, "now do you want me to show you around or not? It's already…" Bill pulled out a pocket watch as large as himself from literally nowhere, "one o'clock."
"Right, right. Let's go." Dipper agreed.
"Great!" the dream demon sounded happier at the agreement and reached out a hand to Dipper; when the boy hesitated and appeared clueless as to why he was holding out his hand, he explained: "what, you want to walk for a few hours? I was going to teleport us but-"
"Ok, ok – I don't want to walk all that way." Dipper said, taking the demon's hand. Bill outstretched his other palm to Mabel and both the demon and Dipper looked at her expectantly as she hesitated; Dipper could see as clear as day that the girl already didn't trust Bill, shocking him since she was so accepting toward everyone.
"C'mon, kid, we don't have forever- well, you don't anyway." Bill coaxed. Giving one last glance to her brother warily, she finally took Bill's hand.

A shiver ran up Dipper's spine as the cobalt blue flame licked up at his flesh, yet didn't burn or scorch him and in fact helped him to relax and even feel slightly tired all of a sudden; his eyes slipped closed for a brief half-second before he snapped them back open and stumbled back, fighting off the sudden sleepiness. Looking around, the trio was surrounded by lush greenery, however Dipper could only see this from the bright light emitting from Bill's triangular form: they were so deep in the woods that the trees were tall and thick enough to block the sun's rays. The heavy sound of coughing broke the trance and he looks back to find his sibling doubled over in obvious pain; wheezing violently. "Mabel!" He closed the gap between them in one large, swift step and he patted her back comfortingly; "what's wrong?"
"Oh yeah, I forgot to tell you that you meat bags react differently to being transported. Some of you end up perfectly fine or maybe even healthier than before; others end up coughing up their lungs or maybe having their arms and legs switching places. Funny really since it's so unpredictable!" Bill told him.
"What?!" Dipper glanced at his sister frantically, preying she was ok. "Can't you help?" he looked back at Bill, deciding to push back the anger that flooded him for later and focus on helping his twin.
"Sorry, Pine Tree, I'm a demon of the mind, I don't deal with all the physical stuff like…" he gestured to Mabel's curled up form. "But hey, at least if she dies, you can finally have your own bedroom!"
"Bill!" Dipper shouted and was about to yell more when he felt a hand on his upper arm. Looking over, he saw Mabel bent over with a hand supporting her on her knees and the other rested on his shoulder; she was panting and her eyes brimmed with tears.
"It's ok," she panted, "I'm fine."
"You looked like you were in a lot of pain, Mabel; are you ok?" his tone softened and she swallowed before nodding.
"Great!" Bill floated in front of the duo and clapped his hands together, "let's get going, then!" He turned and started floating away; dipper was just about to shout and yell at the immortal being when Mabel straightened slightly, shaking his shoulder gently. He looked at her and she shook her head, discouraging him from shouting at the demon.
Ahead of them, Bill smirked to himself.

So is Bill up to anything? Is Mabel actually ok?
Probably xD This isn't meant to be a dark fic and there 200% won't be any main character deaths, but that doesn't mean Bill is going to be completely nice and honest, does it? ;)
I do plan on this being a BillDip fic, just to warn y'all. But obviously not yet.
Next chapter will be them meeting their first monster/creature! Be prepared for terrible jokes!
Anyways, leave kudos, comments, etc please? I'll love you tenfold if you do!

Also let me know if you want more side characters showing up (E.g. Grunkle Stan, Wendy, Soos, etc.) and if so, who? Or let me know if you want less side characters, maybe? I plan on getting them in occasionally.