He was just one of those guys, his name and rank was MSGT. Arnold Fields. But he said he really didn't talk to Carlos that much, Carlos just helped him a little bit on his worming technique and detection of tripwires and the like.
It was kinda cool to hear his story, then read the book, and see that he was one of those guys that the book talked about (Marine Sniper: 93 Confirmed).
EDIT: If you remember the part where Carlos took 3 rookies out with him, my Grandfather was one of those guys, he saw that kid that almost shot a civilian, and said Carlos got pretty pissed about that.