Here is a link to Nathan's character forms. Both are pulled more or less directly from The Plot Thickens by Noah Lukeman, and I strongly recommend picking up a copy of that book for all writers out there, aspiring or otherwise.
Physical and personal life: here
Personality traits: here
Nathan is a pretty mellow guy. His loves in life include his gun, his cigarettes, pickles, and winning arguments. He's a little elitist and controlling, but he's incredibly good at what he does. He's not conceited and doesn't let pride or emotions get in the way of his work (well, unless Sonya is directly involved), just so long as his training or instincts tell him what to do. Otherwise, he's a little lost.
Nathan is fiercely loyal to anyone on his good side, but even that is a little warped; basically, if he knows you, and you haven't held a gun to his head before, he'll save you. Even if he hates you. That said, he will not hesitate to throw anyone else under the bus if it means saving someone more important. He has a habit of rationalizing after the fact, especially if it involves a mistake.
He cares deeply for his country and would go to any lengths to protect it. Unfortunately, many of his friends don't share his ideals, especially as the storyline wears on. He probably cares for Sonya more, though, so that's pretty redeeming.