Jeffrey Hunter goes with David Ward like peanut butter and jelly.
...well, not really. David doesn't like Jeffrey, and J-dawg only tolerates David. But you get them together at the beginning of the game, and they make a decent pair of companions.
Jeffrey comes from Greenhope, which is (about) as country as you can get for Mars. He's a nicer guy than David and seems much happier to be serving under a technomancer than his colleague.
Jeffrey rocks sniper rifles and assault rifles, making him a great ranged combatant. That said, he also seems to love bashing enemies in the face with his rifle rather than shooting them half the time.
I recommend keeping him on Offense mode. There's no reason for him to play Defensive, as he's one of the best damage-dealing companions in the game.
Jeffrey gives a passive bonus of +2 to Physical Damage from Gunshots. He doesn't have an Ally bonus, though.
Spoiler alert - Jeffrey does help you with a quest to root out a spy. Hooray!
But he's another companion that doesn't stay with you the entire game. Sad Christmas.