Marvel is kind of like the Invisible Man's sidekick. Except he's terrible at it. He abandons the IM and still, he gets pretty nicely rewarded. That's kind of an odd moral.
Marvel is something of a stock character when we first meet him. He's the poor, homeless, jobless wanderer – in other words, he's a tramp. He wears shabby, old-fashioned clothes, like his "obsolete hat" (13.39), and he has buttons replaced by pieces of string. The narrator goes so far as to tell us that he does everything in a leisurely manner (9.2). He doesn't seem to like work or excitement. Typical homeless guy in fiction.
He definitely doesn't like working for the Invisible Man. He carries his stuff, including the money the Invisible Man steals in Port Stowe, but this isn't a super fun arrangement for him. So it's no surprise when he decides to jump ship.
But here's the thing: Marvel ends up successful. Since the police can't prove whose money he has, he gets to keep everything that was stolen by the Invisible Man (Epilogue.2). Then he gets even more money for telling the story of the Invisible Man (Epilogue.2) – was Us Weekly around then? That's how Marvel is able to rise up from being a poor tramp to being the owner of his own bar (which is named after his old boss, the Invisible Man).
Marvel might not be any less sketchy than the Invisible Man, but guess what: Marvel doesn't get caught. Aw, snap.