Not just Alcor...

The return

Dipper Pines was an ordinary boy, well he was when he arrived in gravity falls but after he came back to California. He grew taller, his hair became blonde, and most importantly he gained a whole other personality that he didn't have before, and this is what made him unique. Dipper was now an extraordinary boy, with two identities, a new person to everyone else, a new personality, it wasn't his personality although. Only his twin sister, Mabel and few others back in gravity falls knew what it really was, Bill cipher. During the past summer, Dipper had been called many things because of this change in personality, eccentric, interesting (that's what Dipper had decided to go by), a mystery, a hero, and even a demon, but most of all "the boy from dimension 3". That was what Mabel always referred to it as, as if she hadn't heard the nightmare realms name.
It was something of a relief that no one unwanted found out that cipher was still alive. At least not yet anyway. Mabel liked the secret identity part, she said he was like a superhero. 
The bus ride home was long but not unbearable, he sat in the back with his backpack slung across his chest while Mabel sat between him and waddles, the pigs head resting on Mabel's lap. Dipper had his head on the window watching the trees and scenery go by, the bus went through a tunnel and dipper caught a glimpse at his reflection he hadn't seen it in a while. Instead of the 12 year old boy that arrived gravity falls he now had a blond to a brown fade which was oddly growing naturally, his skin was paler and most importantly his eyes were extremely different. The left one was an unnatural yellow with a black, cat like slit for a pupil and the right one was even worse it was black with a white slit pupil. He actually hadn't told anyone but he had gone blind in the right eye. "I can't believe I'm finally out." Bill said in awe. "It's all thanks to you and shooting star."
"Well enjoy freedom while it lasts 'cause he'll be after us." Dipper added averting his gaze from their reflection.

Once the bus had rolled to a stop, the two siblings (and pig) jumped off and walked down the road to their house "what's for lunch, Brobro?" Mabel asked once they had reached their house. "Um... is mac n cheese okay?" Dipper replied, "Wait shit!" dipper stopped walking. "Mum and dad still think I'm dead!"
Mabel also stopped and face-palmed at the sudden realization. "Oh man Brobro how I overlooked that detail"
"Wow you really screwed that up Pinetree!" Bill's voice was full of mirth. "Shut it bill!" dipper hissed. He looked around to make sure no one heard him talking to thin air but nobody was around.
"So did you two plan on telling them?" Bill said once they stepped inside. "Uh i don't think we did, but then again neither did you." 'You didn't plan on telling your parents that you were a freak' Bill laughed. 
"How about we just let them think you're still dead," Mabel offered. "You mean hide and lie like we did back in gravity falls?" dipper scowled.
"Not going to work shooting star" bill said as Mabel lent on the doorframe. "We can't lay low in California," Bill continued.
"But they will question the fact he's not dead, and they won't exactly keep that information away from the public eye" Mabel stated. "Well maybe we can convince them that we weren't dead but in fact kidnapped," Mabel suggested. Dipper rolled his eyes and began listing on his fingers. "And how are we going to explain that they can't tell anyone he's back, the sudden mood swings," he smacked her hands with her fist. "oh! And what are we going to say when the whole population of gravity falls comes a-knocking, armed and claiming their son is an all-powerful demon!" 
"Not forgetting the fact we're now blond, a foot taller and have the ability to merge into a separate entity named Alcor cipher?" bill added. "We'll say you dyed your hair," Mabel said trying to resolve some of the hypothetical questions. She began positively "this is- we should have never left gravity falls..." 
They all froze as the door suddenly opened, scaring the twins, both parents stood in shock, their mother dropping her bags to cover her mouth. Her eyes glossy, focusing on dipper with her mouth agape. "Oh my god it can't be true?!" she exclaimed swiftly, she threw herself towards her son and enveloped him in a hug. "Oh my gosh it is, you're here?!" "Mom I was-" he sighed. "It doesn't matter where you were!" she cut him off holding him tighter "you're here, safe! Oh my god, you're alive!" she squeezed him tighter, tears dripping from in her eyes. "I've missed you so much Dipper".
"Uh mason?" his father spoke "you're blond?" he asked with a questioning tone. "Yes sir." he said "oh my goodness" "dad" dipper muttered. "You got taller," his father, noted, "your eyes" his father looked at him more intently than anyone had ever done. "My eyes..."
'Oh another detail we forgot' bill said telepathically.
"What about my eyes dad?" Bill said darkly. His father furrowed eyebrows "are those contacts?" he asked. "I um... yeah" dipper said. "so in the time you were 'missing', you dyed your hair and bought contacts" he said raising an eyebrow, "instead of coming home? " "Yes?" dipper replied, "and?" Bill chimed in. "don't give me that bull attitude Dipper, you go missing for 10 months, your family believes your dead" he waved his hands in frustration "no effort to tell your family you're okay" he tried to steady his breath "you then just show up like this" he gestured to all of him.
"But dad I'm here aren't i?" bill took over again "why can't you just be happy I'm safe?" "It's not that I'm unhappy, this is just really inconsiderate!" his father exclaimed, "you basically left us!" dipper's mother stood beside the raging man trying to calm him "i was terrified that I'd lose you" the man started tearing up. "Just go to bed, both of you!" he snapped.
The twins headed upstairs.
"Darling, breath. It's okay. He's finally home." Mrs Pines rubbed his back.
"He was reported dead by your uncle, by the town sheriff and countless witnesses, then he turns up again ten months later, looking like-"he gasped for air. "Like that! " Mr Pines moved away, "it's just what happened to him during those Ten months? And why not a word from him or from anyone for that matter?" "Well he's back now... we can ask questions later, let's just be happy for now" Mrs Pines said softly.