As far as I know, Bernkastel's origins in Higurashi are unexplained, though given as I'm writing this before the second season of Gou has aired, that may end up being uncanonical. A Frederica Bernkastel that looks identical to Rika (down to the clothes) shows up in the manga version of Massacre, but anyone who's explicitly Bernkastel doesn't show up in the anime (the adult Rika at the end of Festival Music could have been her, but that's fuzzy) and her origins aren't explained in that manga snippet at all.

June 16th, 2021

Rika didn't like the Sea of Kakera.

Firstly, because every time she opened her eyes to that blue-black void, that purplish nothing filled with dead-end stars, she knew that she had died, she had failed, that her friends had fallen into tragedy once again and she'd been unable to save anyone, even herself.

Secondly, it was because she wasn't alone in that void.

She had been, at first. In those first few worlds, when less than a dozen fragments sparkled and died around her, it had just been her and Hanyuu –who barely counted as anyone, even by her own admission– thinking together, trying to come up with ways to get out.

But as more and more fragments formed, Rika's consciousness began to splinter. There was Rika, the ten-year-old girl with ten-year-old thoughts, and then there was also herself who was starting to think things like an adult, and even though she knew she needed to, Rika didn't want to think like an adult. She was still clinging to her childhood, clinging to the hope of living a normal life with her friends, and she pushed those grown-up thoughts to the back of her mind whenever she could, where they festered and grew.

It was sometime around one of Rena's fragments, Rika thought dully, that she really began to give up. Seeing her friend like that, shrieking and insane, knowing that Rena charred her and all the rest of the class to ashes –that broke something in her. She stopped clinging to the vanishing threads of her childhood and welcomed the shroud of adulthood that she'd pushed to the corners of her mind: and she found them changed.

She found herself talking aloud in the Sea of Kakera, puzzling things through to herself as Hanyuu faded into the background. The more she talked to herself, the more those thoughts weighed on her mind, until eventually they got to be too much and she shoved them away again –as much as she could. She couldn't accept that burden, taking it on in bursts as her voice would deepen and she'd spit a few words of prophecy or despair at one of her friends, or Akasaka, or whoever else she was trying to convince.

Rika cut herself off from her adult side, slamming it behind gated walls, and only let it peek through when she needed that authority or inscrutability, like popping a mask on and off her face. Most of the time, she kept that mask, that façade, hidden and buried, pushed away from herself, where it waited in the Sea of Kakera.

And there, it grew.

The specific occurrences didn't really matter to Rika –she kept cycling back and back to the same point, and the other differences, the only markers of time that mattered, were the new things she tried and how they'd failed. So she didn't actually know when it was that she died, again, and came to the Sea of Kakera, and she wasn't alone.

It was a little girl, a little girl just like her, with Rika's face and hair and eyes, but with something deep and sardonic and adult in her gaze.

"Who're you?" Rika had asked, and the other little girl had giggled in the voice of an adult woman, low and mature.

"You mean you don't know?" she'd drawled, and Rika had frowned.

"What's your name, then?"

The other girl had shrugged, her eyes glinting, as though she was inviting Rika to decide.

Rika had thought about adults, and her father, and his favorite crates of imported wine, which she had recently cracked open for the first time to try and numb the pain of her endless existence. The taste had been dark, acidic, with a hint of something like fruit –something that made her mouth twist, something pungent and adult, something only a grown-up with no tastes for sweets or good things could enjoy.

"Bernkastel." she said, thinking back on the label of that wine. "Your name is Bernkastel."

"Bernkastel, then." the other Rika said, making as though to dip in a sardonic curtsey.

She was always like that, so haughty and smug, like she knew many things that Rika did not. She'd mentioned once that she couldn't leave this plane of existence, idly flicking a finger against a lost fragment as it shimmered and spun into darkness –all she had to do was watch Rika's efforts in all the worlds, reorganize the fragments and stack them together like books as she tried to find patterns, or some sniff of entertainment.

Usually entertainment.

Bernkastel lived to be entertained. She hated being bored, and she would needle Rika relentlessly, poking and prodding for no other reason than to get a reaction. She was useful, and smart, but she was so mean, like living in the sea of fragments like this had never taught her any manners or any sense of consideration for other people, not even Rika. She didn't care about how much it hurt to see Rika's friends suffering –to Bernkastel, they were like characters in a book, and the bloodier the deeds they performed, the more entertaining they were to watch.

No matter how often Rika would tell her to stop, to not idly describe her friends and all their suffering with such disinterest, all she ever received was a sneering, unapologetic apology. Sometimes she wondered if she, herself, was also just a pawn on a chessboard for Bernkastel, if even Rika, who had been the one to create Bernkastel's splintered existence in the first place, meant nothing to the witch.

She hoped she would never have to find out.

9.53 AM, USA Central Time