I had kinda toyed with the possibility that Charlie has been seen, but not recognized by anyone. I was fond of thinking he was JoJo for a few moments until I remembered JoJo was an Unaroyal caster. Then again, he could still be both, using Carnymancy to 'bend the rules' so that he can be both a Ruler and a unit that belongs to another side.
Seems kind of unlikely though, since TGMTTA would likely have ferreted him out (even with the dish, or perhaps moreso because of it). Same reasoning goes to most casters that have been to MK. Of course, he could be pretending to be a non-caster, but there really aren't many options there since GK has virtually 0 living prominent units at the moment. There is Maggie, Sizemore, Wanda, Stanley, Parson, and Zhopa. We can fairly soundly eliminate all of those as possibilities. We can also eliminate all JS units, as they're either dead, not prominent, or fairly certainly not Charlie. Other than that, we only have a couple of other known characters. He might be one of the random Carnymancers that we've seen running around, posing as a Barbarian or belonging to another side (maybe his carnymancy lets him rig which portal he comes and goes from?), but it is hard to believe that TGMTTA wouldn't have been able to figure out something like that.
My prediction is that he'll be more along the lines of Mr. House from Fallout: New Vegas.
I do enjoy the idea of him posing as a Carnymancer or one of the other Casters running around MK, and with the dish he may well be able to avoid detection, so that'd be cool.