Given the concept art of him from Heroes of Dragon Age where he's wielding a maul, I'd say he's an old warrior (and obviously an old friend of Endrin's). Does he say anything to a DN siding with him in convos during APoHK, do you know?
House Harrowmont itself is known to be as old as Orzammar.
Hmm....I know he asks the DN what really happened for Enderin's sake, and he was the one called to Enderin's death bed while Bhelen was repudiated, and he is the one who tells the DN how to get the shield and claim glory (Enderin and Harrowmont were obviously setting up the DN for more prestige there.)
Talking to Nerav Helmi as someone other than the DN, she outright says the DN was Enderin's favorite son, so it seems to me that Enderin and Harrowmont were colluding together to try and get the second child on the throne if they could. But I'm currently playing as a casteless dwarf for now, wanted to introduce Morrigan to my mother (and ironically got some content in the Brecilian Forest I had never got before in the gazillion playthroughs I've done of this game), and I don't remember off the top of my head if Harrowmont says anything or not. I think he does, but I can't remember.