Marcus is the store attendant and customer assistant of Perseus. His name is revealed in The Ballad of Gay Tony during a Random Encounter with Daisie Cash-Cooze, when she reads a Bleeter post by movie star Chris Hunt. Marcus often compliments Niko Bellic if he is wearing clothes from Perseus, and will somewhat insult him if he's wearing anything else. He also claims to have a nipple ring, which is mentioned in GTA IV if the player bumps into him.
Although Marcus can be found at Perseus, pedestrian models that share the same design cues and voice actor can be spotted in Varsity Heights, The Meat Quarter and Middle Park West, occasionally dressed in a more informal sleeveless top and usually speaking in a less formal tone.
Marcus is most likely homosexual because of the way he talks, as well as his reference to a nipple ring that he wears.
If Niko shoots or attacks Marcus, he will have the female pedestrian scream animation.