Marcus is the brother of Emma, former friend of Number Five. He is the leader of an organised crime group based in Miami.
Emma tells Five that her brother is a big deal in Miami and that if he were stealing wallets for someone, it would be Marcus. Despite his activities, he shows concern at Emma's involvement in similar because he feels she is too young.
Emma and Five begin to build a reputation for themselves around Miami, such that Marcus soon hears of it and warns his sister to stop before she gets into trouble.
Despite this, Emma wants to get into more dangerous territory and goes with Five on a task for Ethan, planting bugs in a warehouse. A dozen guys surround them and attacks Five, he responds with an outburst of telekinetic energy which blasts them away. Ethan, by coincidence, was one of the group and suffers from a broken leg and cracked ribs, Emma is concerned for her brother and turns against Five (Five's Legacy). It was later revealed that the attack was staged by Ethan but he did not know that Marcus would be involved.
Ethan spends a while in the hospital and decides to remain in Miami while the rest of his family move to Tampa. The Mogadorians, however, still keep tabs on their whereabouts.
When Commander Deltoch brings Emma to West Virginia, Fives decides not to kill her, despite his anger, and suggests that her brother, Marcus may be useful to them and to keep Emma prisoner in order to obtain information from him. (Five's Betrayal)