[@6slyboy6]

You can definitely be an Arbitrator(Enforcers are Arbitrators recruited by the Inquisition, but this group is not of the Inquisition) if you wanted to, but your character's backstory makes it sound as if Robert would be more of a Mercenary type. 

One thing to keep in mind is that Arbitrators tend to be fanatically lawful-neutral, and can sometimes act as both judge, jury and executioneer. They expect an almost stoic devotion from their members. Think of Judge Dredd if you need inspiration.

I think a mercenary class might suit Robert, seeing as he is such an unsavory character. It's also easier to explain why he possessed such a rare weapon like the graviton gun, as you can say he found it when looting/scavenging, unlike the Adeptus Arbites, where they often have very standardized equipment and deliver all found goods to the administration.

As for Robert being [@mattmanganon]'s leash holder, it probably wouldn't make much sense unless he was a member of the Adeptus Mechanicus, as I can think of few Imperial groups who would drag around mutant monstrosities(quite popular among the Dark Eldar though) and actually have the legal documents to avoid getting into immediate trouble. Another bonus of the Adeptus Mechanicus is that they usually have access to very fancy equipment as well, being hoarders of technology.