1) Do you expect him to fight? If no, all you need is "Sir Picks-a-lot, meticulous quiet man who talks to himself while picking a lock. Theivery +X" (where X is something appropriate to the level of the need)
2) If you do expect him to fight, is he a seasoned fighter compared to the PCs? If no, all you need is the above, plus call him a minion (1 hit point), and give him a basic attack "Pick Stab +X vs Ref; Y damage and slowed until end of targets next turn" (Yes, i'm just making this stuff up). And refer to the DMG for level appropriate, defenses, attack mod, and damage (consider him/her a skirmisher). Or maybe he is exemplary compared to the PCs (making him an elite or solo)
3) If you do expect him to fight and he is to be a worthy adversary, either a) take a monster manual creature like a goblin and reskin it to human (or whatever race). Or b) create your own (similar to #2 above, expect give him actual hit points). Or c) take a basic creature and apply a rogue template on him.
So, you already said you think he/she might be involved in combat. So that leads to the next question -- do you expect him/her to be a capable fighter compared to the PCs (this determines whether or not he/she is a minion, or maybe even an elite)? And around what level do you expect him/her to be (within a level of two of the PCs)?