I've been saying for years that the most frustrating part thing about Quinn is that he almost works. On paper, he has a fairly interesting backstory, the son of nobles, he ran away and joined an assassin's order. After he learn what he could, he left the order (the Talon's not being an order that you could just do that) and joined up with Tollus. He joined Tollus because back then it was essentially a hippie love commune that was a robber's gang. When Tollus started to enact his grand plan, Quinn left once again and was chased for it until you rescue him. Think about it, Quinn is the only person we know of that ever-left Tollus's cult. This paint the picture of someone who is a bit cowardly, foolish, but possessing a strong moral compass.
Unfortunately, the game doesn't really play into his backstory. Sure, Quinn has some extra dialogue when encountering cult members but it's very sparse. There is no quest that ties Quinn with his history with the cult. I feel like they're missing some really low hanging fruit here. Have a quest where Quinn has to confront one of their friends/lovers from the cult, who is extremely hurt by Quinn leaving the cult. Have the inciting incident where Quinn left be the plan to kidnap a virgin (Cait's sister), boom, instant pathos. Have cult members react more hostile with Quinn in the party. Let Quinn have talks where he can discuss high level operatives in Tollus's cult such as the Faceless Blade or Master Whisper. Quinn should at least have a clue on who the Faceless Blade is, and we know for a fact that he knows Master Whisper.