Interesting theory
http://finance.yahoo.com/news/mind-blowing-theory-monster-stranger-134549653.html
The show has already revealed that Eleven is the reason the monster, dubbed the Demogorgon from "Dungeons and Dragons," exists — it's the result of the torture placed on her due to Dr. Brenner's (Matthew Modine) experiments.
But what if their connection runs deeper? This theory points out a lot of interesting connections between Elle and the Demogorgon, which the theory suggests is actually a manifestation of all of her fear and terror and therefore a part of Elle herself.
Among
its many references, "Stranger Things" mentions the "X-Men #134" comic, in which Jean Grey is overwhelmed by her dark side and transforms into the incredibly powerful and evil Phoenix. The Demogorgon could be the Phoenix to Elle's Jean Grey.
Most notably, the Demogorgon's Upside Down world and Elle's real-life world are mirror images just as they themselves are mirror images — shown most explicitly in their final confrontation when Elle raises her hand to the monster and it raises its own hand in return (see the photo above).