My Stranger Things 5 Theories

Another person in the Upside Down

My theory is there is another person in the Upside Down, either another test subject or the person who somehow created the Upside Down (Or both)

There is no way Will survived in the Upside Down with the demogorgons and demobats and vines and stuff. 

I think this other person in the Upside Down, helped Will survive and that's how Will survived the Upside Down.

Also I already wrote this, but when Vecna's body just isn't there at the end of S4, it's because this person in the Upside down saved him.

I think it would also be cool if this person was like connected to the Upside Down somehow and could like control it, and created the monsters.