Not much is known about Marco before the apocalypse, except he lived either in or near the city of Tijuana. It is possible he was involved in gang activity.
Marco was the leader of the Los Hermanos Gang. It is unknown if he was always the leader or if he formed the gang after the apocalypse. He seems to be a man who tolerates agreements with people outside the group only for a short period of time, as he was willing to move against the people of La Colonia in order to remove their control over the local pharmaceutical supply.