Marcus lived in Maplecrest with Filia and his wife, Amelia. He allowed Filia to adopt Dog (and Samson, unbeknownst to him), and at some point afterward he cut ties with the rest of the Medici. A year afterward, Marie appears at their house to kill them, but Dog and Samson fight her off. She allows them to live so long as they leave the city immediately. While Samson apologizes for his deception, Marcus thanks him for protecting his daughter, and considers him part of their family.
Later that day, his brother Vitale visits with Ottomo to see if they had survived Marie's attack as every other Medici in Maplecrest had been killed earlier. Vitale notices them packing, and accuses Marcus of betraying the family for the Skullgirl. Without allowing for explanation, Vitale shoots him and Amelia, presumably killing them.
He seemed to be a kind and upstanding person who cared about his family, ceasing communication with the Medici assumedly to distance himself from their criminal activity. He also welcomed Samson into the family for saving Filia from Marie despite Filia's earlier remarks that Marcus disliked parasites.