Marcus is the store attendant and customer assistant of Perseus. He 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, hinted at by some of his dialogue.
If Niko shoots or attacks Marcus, he will have the female pedestrian scream animation.