Antonin is a minor courtier in the Doge's Palace, on the upper floors. Scarlett will meet him as part of the Shady Assistance quest. He'll be found in his room on the upper floor of the Palace, where one of the palace's Marauders who serve his Guardsmen standing outside the door. He's been partying with old friends Venus and Vanity from the Ambrosial Lotus.
Shady Assistance - Help Antonin discreetly dispose of an inconvenient corpse.