To me Eleven is a traveller from another world. She's a princess, who got lost in Upside Down and when she found a way back, she opened a door for the supernatural to seep into the reality. But in the same time Will is the counterpart from our dimension.
The series doesn't explain exactly how all of this became true, but after two seasons it's clear that these two children are the key for whole thing. Eleven closed the portal, but she didn't remove the evidence. The Upside Down will always be there, at the background, lurking and being ready to burst out and haunt the young audience. Although the game isn't about the youngest members as the cast adults are in hugely role.
In the second season She... Chief Hopper played so important role on keeping Eleven from solving the problem. He isn't by anyone standard a good dad, even though he's trying his very best. But that's the thing when you're the sole head of the investigation and you two families to look after, as Eleven still probably won't know most of the things Hopper were doing when he was out from the hut.
Why is that we'll have to give the visitors from another realm an access to television? He did a good thing by teaching eleven vocabulary, but at the same time he did damn awful job on engaging it. Instead of seeing El's brilliance, Hopper dulled it down. In other words, if he'd given some room for Eleven to proser, he wouldn't have ended digging a cornfield at 8 pm and realising he was awfully late.
By Hopper's actions he caused Eleven to run away, find his sister, learn about her mother and got himself almost killed if Will hadn't saved him. Poor Hopper, but then again the Depart of Energy's boffins weren't doing a magnificent job either. They were trying their best to contain the situation they couldn't even beginning to understand.
All progress was made when the paranormal entered the picture, but even then the scientist were firmly sceptical even though they'd evidence literally squirming in their basement. The poor squad of redshirts had no chance of surviving with the d1ckhead scientist running the comms. They were probably best of the best US can offer to DoE, but even then they weren't enough. The beasts most lethal weapon were their alien physiology to our norms. But we don't know if the big bad is the worst opponent.
All we know about this gargantuan beast that it's very hard opponent to kill. But in their world, he might not even be on top of the resource pyramid. There might be scarier things prowling the wasteland. What did the DoE really learn about the Upside Down?