This isn’t canon, but Mod A likes the idea, so …
It had taken an unusually long time for Mabel’s soul to reincarnate, Dipper thought to himself. He hadn’t been complaining when her soul had stuck around with him for a few months after her death, but the longer she’d stayed, the harder he knew it was going to be for him to let her go again, to let her reincarnate at all.
Thankfully, she’d finally been pulled into a human body again, and after watching over her new mother for nine months, Dipper was there (albeit invisible to all present and in the Dreamscape) as she was born again.
It was a little bit weird to watch as his twin sister was born, maybe, but nothing about Dipper’s life had been normal for years.
So he watched as the doctor delivered a screaming baby boy with Mabel’s soul and handed it to her mother, then idly noted as the doctor delivered and presented to the mom a smaller baby girl—his sister’s new twin.
A pang of jealousy struck him, as he thought of his sister, his twin, growing up with someone else as their twin, knowing someone else as their sibling, but he pushed it down (that was demon thinking, that was like Bill, just be happy for her, at least she won’t be alone).
Dipper smiled as he watched the new mother coo at her babies, watched as Mabel’s new twin sister’s arm flailed and grabbed at boy-Mabel’s tiny hand. The sheer happiness and warmth from all the auras in the room was blanketing, and Dipper stopped suppressing his ‘aura-vison’ (as Mabel had taken to calling it) and watched the warm oranges and bright greens of love and contentment spread across the room, radiating out from the new family.
He could sense the bright light of Mabel’s soul, pulsing and radiating happiness, and his smile grew to a size impossible for humans. And then he caught sight of her new twin’s soul and did a double-take. He was sure he knew that soul from somewhere, was certain he’d seen it before. But it wasn’t Stan’s (his grunkle had reincarnated years ago, and besides, this soul was different) or Henry’s (which he knew without looking had gone elsewhere, to a young Mexican family on the other side of the country), or even Candy’s or Grenda’s, and no one else whose soul would have seemed so familiar had died yet.
He poked at the soul, cautiously, hoping to discover something about who they’d been, and was flooded with memories of past lives.
A Native American tribe. A band of assassins. A candy shop in a small town. A small but colorful California town. A million past lives that he knew he’d seen before, that felt awfully familiar yet distant.
And then—and then—
He saw a flash of the Shack with the sign intact and slowed down the speed of the past lives flashing before his eyes. He watched a life he knew, a life he’d lived, a life growing up with Mabel, a life where he took a vacation to Gravity Falls when he was twelve and still human. He watched himself finding the Journal, investigating the mysteries of the town, facing Bill in the Transcendence.
And then the memory skipped. Right after the boy—himself—Dipper had been possessed, it skipped again, to a woman growing up during the Transcendence, battling depression, and jumping from the top of an apartment building less than a year before the present.
Dipper—Alcor—stumbled back, reeling. How was that possible? How had his soul been reincarnated if he wasn’t dead?
He found himself opening his mind to the knowledge of the universe, making use of his near-omniscience to a degree that he hadn’t in decades, and had never tried to use intentionally, but in this case, he had to know why his soul had just been reincarnated with Mabel’s when he was still alive, right here, and his soul couldn’t possibly have left.
He flashed back through knowledge of decades, looking back to the day of the Transcendence with his new omniscience at his disposal. There, Bill trying to carry out his plans, and there, himself enacting the spell that he and his allies had discovered, the one that would end up defeating Bill. He watched as Bill’s energy dissipated in great bursts as the spell hit, and then he paid close attention as he watched what had been the last moments of the soul that was now Mabel’s twin.
Bill Cipher headed straight for Dipper, to possess him. Alcor watched as Bill forced his way into Dipper’s body, taking control of it, watching Dipper and Bill’s souls fight for control.
And then—
Dipper’s soul flickered and wavered and darted from his body. Dipper’s life flashed out like a birthday candle on one of the triplet’s cakes, blown out in an instant. And in its place, he watched as Bill’s soul, Bill’s life and power, were bound to Dipper’s body, pulling it into the Dreamscape. For a moment, he darted into Bill Cipher’s mind, watching as centuries of being a demon were overridden with the color-filled memories, thoughts, and experiences of the human boy whose body and life he’d stolen. Bill’s soul recolored itself to match Dipper’s, but it was only the shell of the soul—the core of it was still entirely Bill’s.
There was no pretending now. At his base, at his center, Alcor was Bill Cipher. Admittedly, he’d been reshaped, remade to believe he was Dipper Pines, but Dipper Pines had died the second Bill Cipher had seized control of his body.
Alcor flew back, flinging himself into the depths of the Dreamscape in his horror, in his attempt to get away from it, but he knew now, he remembered now. As hard as he’d tried to avoid acting like Bill, becoming Bill, he always had been. He’d just never known it.
A frantic, inhuman laugh tore from his throat. The situation was too absurd. Everything he thought he’d lived, everything that he thought defined who he was, was stolen from a boy who’d died decades ago.
He could feel the gold of his irises swelling to envelop his whole eye, his wings flaring bigger to cocoon him in his confusion, could feel the shimmering tears leaking from his eyes.
He was a fraud, a fake. He wasn’t and had never been Dipper Pines.
But hadn’t it been so nice to pretend?