Ezra Stern:
We may know each other, even if in passing. I served in the areas as well as a doctor.
Hmm...it's possible, if not socially, then perhaps professionally.
It depends on what you've been up to in the area, I suppose; Harker is a bit of a dispossessed Soldier of Fortune, so he's got around a bit and fought where the pay is good enough, rather than for King and Country. He's really more of a guide than a soldier though; he likes the wilderness better than the city.
@Lord Byron: Same goes to you as well, then. Harker is a working class man, so if you do know him at all it's because you've worked with him in some fashion, but it's unlikely that you'd recognise him by name, unless something extraordinary happened to bring them closer.
It's possible that he was part of an expedition or unit either of you've operated with.