is there a way for Esteban to join your party? He seems like a cool dude and he could be super useful.

Yes and no. Yes - if you backed the game via kickstarter (exclusive deal for backers). No - if you got the game from any other source. Since you are asking here the answer is most probably no ;)

That's too bad. Thanks for the reply!
It is crazy to think these guys made the game with 77K. It plays a lot better than much bigger titles

Huh, it would be interesting to see if it actually works.

The reason I'm skeptical is because I "modded" him in by copying the XML files into a new "language" directory, and then changing the scripts so I could get him through the dialog.

When I reached that point, I asked him to join me and he did indeed accept, and did show up as a regular party member (just a soldier with a different bio); however, shortly thereafter he mysteriously left! I got a message saying "Esteban has left" or something and he showed up as "missing" from there on out.

So I figured the devs had put in some sneaky check for to see if you have "backer credentials" or something of that nature, causing him to leave.

But if someone gets him in and he actually stays, that would be interesting. Just for the sake of completeness. And to finally see his scripted Mexico sequence play out (which from reading the XML is pretty funny).