Maybe this is being discussed elsewhere, but I can't find it....where do we think Carl came from? What group? Who are his parents? Are he and Alex rebel leaders together?? I know there's more here...

Well we don't know where he came from originally. But he was in the cage across from Sawyer forever ago when Lost started the third season. He escaped, let Sawyer out, was promptly caught, walloped, and we haven't seen him again until tonight.
Although Alex did break in to the rock moving camp and scream and yell for Karl and asking for Ben. I think she said something about, they killed her boyfriend and they were going to kill Kates boyfriend too. Something like that.

There's not much more. Carl (or Karl) was in the opposite cage that Sawyer was in at the beginning of the season. And then he is just alluded to by Alex. I would rewatch those eps.