Jean-Marc Mercier

Jean-Marc Mercier attends Mayor Henri Lemieux's party at the mayor's Saint Denis residence in 1899. Arthur Morgan is among the guests, alongside Dutch van der Linde, Bill Williamson and Hosea Matthews. Mercier stands with Lemieux, Evelyn Miller and Ferdinand, holding a glass of champagne. As Lemieux's deputy mayor and lieutenant, Mercier moves in the mayor's circle, though he does not share all of his employer's methods.

After Lemieux summons Arthur Morgan or John Marston to his home at night, Mercier admits him and assists in finding Professor Shiftacre. Lemieux wants Shiftacre frightened into silence over forged paintings in Saint Denis. When the professor has been dealt with, Mercier voices his objection to the mayor's corruption. He later interrupts Lemieux as the mayor plans to pressure newspaperman Hector Fellowes into financing a library, saying that he is leaving Saint Denis by stagecoach and refusing any part in the affair.

Following the intimidation of Fellowes, Lemieux tells Arthur or John that Mercier intends to expose both the mayor's dealings and the protagonist's criminal past to the press. The protagonist confronts Mercier at his house, knocks him unconscious and carries him to the garden behind Lemieux's residence. Mercier openly denounces Lemieux there, while Lemieux orders the protagonist to kill him before walking away.

If Mercier is killed and his body is thrown into the nearby water, Lemieux remains mayor of Saint Denis. If he is spared, Mercier promises to leave Arthur or John out of his accusations and flees the city. He then exposes Lemieux to the press, bringing about the mayor's resignation. Mercier becomes mayor of Saint Denis, and the mayor's office later sends a letter confirming Lemieux's departure.