Mike

Mike is Rob's best friend by default, despite the fact that Rob calls Mike "gay" or "fag" on a consistent basis.

Background
Mike is a nerd, as suggested by fact that in Episode 1, before Rob joins him, he appears to be playing Unforgotten Realms by himself, in what appears to be Rob's basement.

Role in Plot
Mike is the Dungeon Master and creator of Sir Schmoopy's companion, Eluamous Nailo. The only character that is confirmed to be voiced by Mike is Maelstrom.