I too believe that some of Rejet's games take place in the same universe! The events of Black Wolves Saga took place several centuries ago, when the Sakamaki were children, and everything that happens in Dance With Devils, Carnelian Blood and Diabolik Lovers is happening at about the same time. And yes, I'm sure Karl is the big bad in all of this histories 💀.
OMGGGG YESS! The cat people in Black Wolves Saga are never mentioned in any of the diaboys routes unlike races like the Vampires (obviously), Wolves, Vibora, and Alder. It was a little suspicious why there were so few races mentioned in Diabolik Lovers.
It's easy to assume that those four just stood the test of time, and that might be true. However, there was such a wide variety of races in the beginning of Black Wolves Saga, but most of them were either dead or dying, like the bunny boy, Zara Skeens. He's the last of his kind, and the only reason he isn't dead is that he was taken in by Foina's dad, Edgar Galland.
The same goes for the cat boys, Mejojo, Auger, and Julian. I would say the same for their parents as well, but most of them met an unfortunate fate, by the start of the story and now their whole race is slowly dying just like the humans the wolves were killing at the time. Yes you read that right there were humans living in the demon world more specifically Weblin a place for both humans and demi-humans, (creatures who have the power to shift between both human and animal forms.) before Arles made it his life misson to eradicate them.
I'm also assuming their lifespan is not nearly as long as vampires. But it wasn't just the rabbit race human race or the cat race that were dying some wolves were also catching a disease that was wiping out their race aswell (seriously, what's up with Rejet and having diseases decimate entire races?).
The disease was called Zodiva, a.k.a. "Mad Wolf Disease." Some known characters afflicted with this disease were Arles and Rath. However, it's puzzling why we don't see this disease affecting modern-day Diabolik Lovers wolves. My theory is that Karl released this disease to quell the rebellious wolf population; as the wiki states, "For many years, humans, cats, and wolves shared political power, but one day, the wolves revolted, plunging the country into chaos." and "Arles reportedly led a revolt against the monarchy 10 years before the story's beginning and was thus driven out along with the other wolves."
Karl has a history of afflicting people with diseases to defeat them (cough, cough, the founders).
While we're on the topic of founders, we can imply that the person who created Endzeit was Carnelian Brecht from the series Carnelian Blood @vampirerosemary explains it way better then I ever could so I would recommend you read these 3 post!
🦇Carnelian Blood & Diabolik Lovers🦇
🦇DIABOLIK LOVERS IN THE SAME UNIVERSE AS CARNELIAN BLOOD?🦇
Regarding Carnelian Blood in Dance with Devils, it's 100% canon that vampires exist, and they are the main antagonists in the game, alongside the church. Both settings take place in Japan and involve magical beings, but it's a little confusing why vampires are mentioned in Dance with Devils while devils aren't mentioned in Diabolik Lovers. Kanato and Laito make references to Incubus (Kanato literally had the spirit of an Incubus possess Teddy and hold Yui down), and Urie is an Incubus. So, it's possible that devils are a small or lesser-known race in Diabolik Lovers.
It's also interesting that Nesta is canonically the father of Lindo, and the fact that Nesta and Karlheinz share a striking resemblance, from their hair to their clothing style, it's noteworthy to mention Karlheinz's affinity for cosplaying different people, like Reinhart, and having kids who all look different from each other suggests that he might have assumed various identities and appearances throughout history. This idea is even supported by the fact that Kanato mentioned that their father could change his appearance, becoming a man or a woman at any time, and even they don't know what he truly looks like.
You can argue that Lindo gets his hair colour and eye from his mom
But keep in mind, Karl has a track record of making red-haired kids.
Fun facts it is sorta kinda canon that lindo and ayato met. Just look at this cute little screenshot ( 〃▽〃) Source: @itsaboutyourstruly
In my own personal headcanon, vampires age every 50 years. So if what the wiki says is true, that Karl is about 2000 years old, that means he's in his 40s.
In conclusion, Karlheinz is having the world's worst mid-life crisis, and he's making it everyone's problem. 💀