Too be honest, my original concept wasn't much more than 'U.S. Army grunt, deadpan snarker when faced with strange/normally traumatizing things, professional in the line of duty.'...
I never really have an overtly written out background for my characters, just some basic ideas. I like to work things like that out during the actual game, see where it leads and all that.
John did turn out to be a bit less deadpan and slightly more social/considerate than I thought, but I still don't have a real grasp on his personality. I've got some additional background and personality traits I'm working on, but I don't want to introduce too much of that until I'm pretty sure it's something that helps me get inside his head, so to speak.
As for the 3 traits...
1) Loyal to his teammates. They all have the same mission, and even though their motivation may vary, they've all got the same goal in mind and seem to realize they need each other to realize that goal.
2) Professional. He might crack a joke every now and then, but when he needs to get down to business he's dead serious.
3) Stubborn. If something doesn't work the first time, he'll give it another go. He won't accept defeat and keep trying when he's doing something he believes is worth doing, something that serves a higher goal so to speak.