Bill Cipher

A dangerous interdimensional dream demon that was summoned by Gideon in his efforts to take over the Mystery Shack. Bill Cipher is the true main antagonist of the series who works behind the scenes in the first season, becomes prominent in the second season, and (potentially) exists at a meta-level than the rest of the show's cast. He is a jovial and sadistic trickster who makes devilish deals to get what he wants, and claims that something big will soon change the world...

Example of:
Reality Warper