What was the deal with Mike Wheeler in 'Stranger Things 2'?

In the grand tradition of sequels everywhere, Stranger Things 2 had more of everything – more demogorgons, more Will, more Steve being a good guy, more Hopper being Hotper (I'm sorry, I'm sorry, I knew as soon as I said it). 
The one exception? Mike Wheeler.
SEE ALSO: 'Stranger Things 2' review: Binge-watching can't disguise how bad this is
In Season 1, Mike wasn't just part of the gang – he was the guy, around whom much of the action revolved. He was the dungeon master and ring