Eddie will come back in season 5 and he is probably some kind of guardian of the upside down.

Why?

*Eddie started to fail school after the gate was first opened.

*Eddie took mike and party under his belt so he can keep a watch on them.

*The characters in upside down are inspired from D and D which is because the were probably imagined by Eddie.

*But when 1 was sent to upside down by 11 he took control.