I have a pasta headcanon/theory
Slender is responsible for every pasta being here.
The sickness drove Jeff slowly more insane, and the fights and being burnt pushed him over the edge to psychopathic.
Therefore Slender is indirectly responsible for Jane and Nina.
Slender is who kept Liu alive after Jeff tried to kill him, and he also organized the cult that created Eyeless Jack.
The guardian angel who gave Isaac Grossman the box that held Laughing Jack inside? Yep. Slender. Also, the sickness caused his being a sociopath.
Laughing Jack steals people’s teeth and mutilates pets. What does he use the teeth and other body parts for? Decorations for tea parties with the Tooth Fairy? I don’t think so. Who has human teeth and is a possibly mutilated pet? Smile.dog and Grinny. Then there’s the Smiling Cat who is the same way, but is kept alive by Slender’s power.
There’s a theory that the Operator Symbol is a portal that he travels through, and also a portal to Hell itself. This could be how Zalgo came to Earth. Lazari is a demon. A demon may have impregnated Lazari’s mother. If the symbol is a portal to Hell, a demon could have come through and done this.This also links to every other demon pasta, including Kagekao.
Slender makes technology act really weird. Glitchy Red, Lost Silver, BEN_Drowned, Polybius, Giygas, Sonic.exe, Tails Doll, and every other tech based pasta.
In terms of being on Earth, Slender is the oldest Creepypasta, allowing him to have plenty of time to have created them, as well as any others that may have existed in the past. Splendor Man is usually seen as older, but he is a Happypasta. Not a Creepypasta.
The Puppeteer seems to be a manifestation of depression, as most of his victims are found appearing to have hung themselves. Slender Sickness causes hallucinations as well as anxiety, which are both the sensations that appear alongside the Puppeteer.
Squidward and Suicide Mouse were brought on by depression, another link to the Sickness.
Sally’s spirit was kept here through Slender’s powers, along with all the other undead pastas.
Clockwork was also driven insane, and Bleeding Cross took inspiration from the Operator Symbol, as his calling card is simply a set of X’s written everywhere around his victims, whereas the symbol is a circle with an X through it.
Then you have Masky, Hoodie, Ticci Toby, and the other proxies, which are obvious ties to Slender.
Others could be tied in as well.