I'm leaning toward making him a Darkhaft Duergar in the employ of the Laduguer church. His real name (and appearance) is as Hemathon, a spy/agent of Deep Duerra opposed to any demonic influence in the otherwise devilish city.
You see, in my campaign I made him quite insistent on returning to Gracklstugh as soon as possible after seeing the rise of Demogorgon in Sloobludop. That alone made my players curious, speculating he has a role of some importance to play, and I would disappoint them if he "only" was what the module makes him: a small-time arms dealer at the wrong place.
Next, the players dropped the Whorlstone dungeon bash as soon as they found the secret passageway from the alchemy lab up to the docks district, so I want to make something of the
Council of Savant plot to bring madness to the city, and to incite a Derro uprising and/or have Demogorgon reduce it to rubble (I'm guessing even Aliinka and Zubriska, the derro savant twins themselves don't really know).
Having Hemathon suggest to the archpriest of the Laduguer church* in Gracklstugh he's found a suitable party of gullible surfacers that he can use to infiltrate the Council of Savant HQ to check for demonic presence would mean an alternative adventure to the rest of Whorlstone.
What do you think? Any pitfalls to look out for?
*) Grim Herald Brilmara Gloomstorm high cleric of Laduguer
(I thought Morndin would be very old by now, so he runs the church from "behind the throne" with a daughter as puppet)