Sanctuary of the Underdark
Former thief, now insider for the Heroes
Jackson was part of an assassin group set up by Nix. They had made a mistake and rather than outright killing them, Nix put a bounty on the heroes heads, but only told Jackson’s group. Nix knew Jackson and his fellows were highly unlikely to succeed so he used the heroes to his own ends. In exchange for his life, the heroes demanded he act as an insider to get back at Nix. Currently, Jackson’s return is slow as everyone thought he was dead or locked up. He lied and said he got off the hook. Since the Hanging Noose is an exclusive guild, he is doing his own thing.
Jackson is not a bad man necessarily, but he is not a good one either.