Bill hovered in the middle of the room, eye frowning at Dipper.
"Kid, you're reeeeeeeally starting to bug me, you know that? I've got important stuff to go do. Shoo. Get outta here."
"Not this time," Dipper shouted fiercely at the triangle-man. "Not after all you've done, after all these years. I'm not letting you get away. You're coming with me."
"Kid, what happened? Did puberty mess up your whole brain? I'M A FREAKIN' DEMON. See?" Blue flames shot out from Bill, engulfing the room before quickly vanishing. A faint aroma of singed flesh hung in the air. "What POSSIBLE way do you have to reel ME in?"
Dipper reached into his bag and pulled out the journal, flipping it open to a marked page. Bill rolled his eye. "Pfft, books. Who reads books anymore anyways? Nerds. You a nerd, kid? HAHAHA! Oh, right. It's you."
"Ehjrqh, irxo ghprq," Dipper chanted, ignoring him. "Ehjrqh dqg uhwxuq wr wkh khoo zkhuh brx ehorqj-"
Panic shot across the triangle-man's face as he realized what was happening. Beams of light began shooting off of him, as brick by brick imploded into oblivion. As the last brick vanished with an audible 'pop', Bill's eye rolled around violently before snapping shut and falling to the floor with a clunk.
"Yes!" Dipper shouted at the eye, throwing the journal on the ground. He started dancing. "YESSSSSSSS IN YOUR FACE! THAT'S WHAT YOU GET WHEN YOU MESS WITH ONE OF THE PINES TWINS! Oh man, Mabel's gonna be so stoked-"
"JUST KIDDING!" Bill yelled as he popped back into existence right behind Dipper. "That was close, though. You alllllllmost got me."
Dipper screamed girlishly as he scrambled back towards the journal. "OH NO YOU DON'T, KID!" Bill shouted, hitting Dipper in the chest with a fireball. Dipper flew across the room, smashing against the wall. "It's time for me to take you out of the equation."
"Please - don't kill me," Dipper wheezed, blood trickling from the corner of his mouth.
"KILL you?" Bill responded, laughing maniacally. "That's so BORING! Nah, I've got something much better in mind. See, I know a certain secret that Shooting Star is keeping from Pine Tree-"
"Mabel? No, keep her out of this. This is between you and me," Dipper yelled angrily.
"Eh. It's more fun this way. Trust me," Bill replied, winking. Suddenly, Dipper's consciousness was split from his body as Bill ripped away control. "Kid, remember that agreement you signed with me when you were twelve? Because you were so STUPID, I can still technically use you as my puppet. That thing's still legal under demon law, y'know. Like, even a twelve year old should know to read a contract before signing it, right?"
Dipper flew toward him, fist outstretched. "Get out! OUT!" he yelled, passing harmlessly through Bill, still in the middle checking out his new body.
"Alright, alright kid, settle down. I'm just borrowing it, is all. You can have it back when I'm done, and I PROMISE no harm will come to it. Good enough?" Bill asked, flexing his new muscles. "Also, good job beefing up, kid. You must be a hit with the ladies."
"What if I don't go with you?" Dipper asked, frowning suspiciously.
"Well, you'll have no idea what I'm going to do while I'm in your body. And besides, then you'll just be trapped in the mindscape forever. So it's basically like you don't have any choice. OH WAIT! It's exactly like that. C'mon, kid. We're going home."
-x-
Dipper floated nervously behind Bill as they made their way back to the Mystery Shack. Bill opened the door and walked inside, making his way towards Mabel's room.
"Hey now, what are you doing?" Dipper asked. Bill banged on the door.
"Sister - I mean, Mabel - it is I, Dipper! Your brother! May I enter this room?" Bill asked.
"Uh, sure, Dipster. Why do you sound so weird?" Mabel opened the door, groggily. She was still in her pajamas. Without a word, Bill leaned in and kissed her.
"What the hell, man?" Dipper shouted.
"What the hell, man?" Mabel asked, voice cracking awkwardly.
"Mabel, I read your journal," Bill grinned. "You really need to hide it better next time." Mabel's face shot beet red. She tried to slam the door, but Bill shoved his foot in gap. "Hey! Hey. Listen. It's okay. I'm glad it's not just me," the demon replied soothingly.
Mabel's eyes grew as big as saucers. "W-what?"
"I feel the same way about you." He kissed her again, but this time, she responded. Mabel grabbed Bipper and pulled him close, as she started to cry.
"Oh Dipper, I was so scared!" Mabel laughed, tears welling in her eyes. "I felt so helpless and angry about my feelings for you! So many nights up thinking, hating myself, but now? Knowing you actually feel the same way?"
"WHAT. WHAT THE ACTUAL FUCK?" Dipper yelled, waving his arms. "HELLO?"
Mabel and Bipper kissed more passionately, each one clumsily trying to take control. Mabel fumbled for the door, locking it. Dipper floated through behind them, as Mabel forcefully pulled Bipper onto her bed. Bipper kissed her roughly on the neck in response, as Mabel let out a soft moan. She ripped off her pajama top, breasts spilling out. Dipper was reviled.
"THAT'S MY SISTER! HEY! KNOCK IT OFF!" he yelled at the two of them. He received no response. His skin crawled - well, if he still had skin. Dipper wasn't sure what was up with that right now.
Bipper grabbed her breasts, playfully rubbing her nipples. Mabel moaned again, as Bipper slowly slid his hand down her front, into her panties. Dipper flew towards the door, but with a flash of light, he became frozen in place.
"Oh no. OHNO. Bill, c'mon man, this is SO FAR over the line. You've got to stop." Dipper yelled. No response. "For fuck's sake, at least let me leave. Don't make me watch this, please." Dipper tried everything in his power to move, but he was stuck. He tried to yell again, but his voice was gone. He was completely helpless, forced to watch the spectacle unfolding in front of him.
Mabel unbuckled Bipper's belt, ripping down his jeans. Reaching into his boxers, she drew out his stiff cock, staring at it momentarily. Cautiously, she began stroking it, rubbing it slowly from top to bottom, feeling the heat emanating from it. Bipper shuddered, and Mabel kissed again, hungrily, before Mabel bent down and began clumsily sucking his cock.
"Oh yeah, sis," Bipper groaned, "Just like that." He reached back into her panties, rubbing his fingers over her wet pussy before finding her clit. He found a pattern that Mabel seemed receptive to, feeling her slickness. Mabel stopped, panting heavily, and clumsily ripped off her pajama bottoms.
"Fuck me." she said. It wasn't a question. It didn't have to be. Mabel helped guide Bipper's cock into her entrance, feeling it slide in, inch by inch. "Oh god, you're so BIG!"
"And you're so TIGHT," Bipper groaned, slowly thrusting his hips back and forth, feeling Mabel's unfathomably warm pussy engulf his member. Bipper slowly picked up the pace, until finding a comfortable speed. Mabel seemed to approve.
"Oh my god, oh my god, oh my god, oh my god," Mabel mumbled, barely able to form words, "I can't believe this is happening." Her voice trailed off, replaced instead by heavy breathing, punctured with soft moans and encouragement.
"And I can't believe how wrong I was! Tuns out that pleasure is LOTS better than pain!" Bipper replied, breathlessly. Mabel slapped his ass in response. "Harder," Bipper grunted. Mabel obliged, and Dipper could make out his sister's handprint on his own ass, all the way from across the room. "AHHH OKAY THERE WE GO." Bipper kicked it into overdrive, caressing Mabel's breasts as he pounded her roughly, the two of them moving in unison. Even their breathing had synced up. Mabel mumbled something again, but Dipper couldn't hear her. Neither could Bipper. "What?" he asked.
"mmmmmm cum in me CUM IN ME" Mabel finally managed to say, panting heavily. "uhhhhhhhhhm on pilllssss"
With a grin and a groan, Bipper did. Mabel lay panting on the bed, but Bipper's erection had not subsided. "Hey! Still hard!" He turned and locked eyes with the corporeal Dipper. "Like magic!" He turned back to Mabel. "Ready for a round two?"
-x-
The sun was rising, and Mabel lay sleeping peacefully on her bed. Bipper got up and stretched. Dipper still floated, frozen, in the middle of the room.
"Good morning, good morning, good morning!" Bipper said, grinning. Without bothering to put on clothes, he dragged the frozen Dipper out into the hallway and shut the door.
"That sure was fun, eh kid?" Bill asked, rhetorically. "And look, I wasn't lying! Your body is fine. Maybe a little dehydrated, but for letting me use it! Take good care of it, kid. Work out or whatever. You never know when I might be stopping back again! Oh, but one last thing: brx ixfnhg brxu vlvwhu. Bye now!"
And then Bill was gone, and Dipper was standing naked in the hallway. The door opened behind him.
"Dipper? How you feeling?" Mabel asked, smiling.