I assume you mean Eleven, and my assumption is that the Uspide-Down spawned from her psyche, particularly since she vanished with the monster and now seems to reside there.
Personally I think there’s just so much that doesn’t quite make sense about the first season that it’s best left unresolved. How Will is coughing up slugs, how Eleven is only in the Upside-Down, why there was only one of the creatures, why Hopper willingly got in the car, etc. I worry if we dig too deep it falls apart. Also, my wife said that the kid with the lisp apparently couldn’t do any of his voice-overs in post-production bc he got his teeth fixed and he sounds nothing like he did before. It would be a real bummer for him to not be a goofball with a lisp. I also think it loses some of its magic if the core group of boys age up and are high schoolers.