Marco is the brother of Valerio Jimenez. Prior to the events of The Evil Within, Marco Jimenez was a doctor at Beacon Mental Hospital. He soon found out about Ruvik's plans, however, and escaped his vengeance with Leslie Withers, his current patient, in tow. He helped Sebastian Castellanos escape the collapsing city, moving into the woods.
Here, he saw the death of his brother at the hands of Sebastian once Valerio turned into one of The Haunted. Soon after, he succumbed to the darkness himself, and was dispatched by Sebastian.
Marcelo is best dealt with in the same way Valerio is - several well-placed headshots will render him dead with relative ease.