His full name is Marchrius Dennis Rossi. He is the main character and protagonist of the series, and has appeared in all the titles (except for Metal Slug Advance). He comes from theUnited States and has Italian ancestry. Born in Idaho, Marco is a computer expert who particularly enjoys writing computer viruses and programs; one virus he wrote managed to penetrate all of the US Military's defenses and nearly caused the launch of nuclear missiles. Marco graduated from Military's Special Technologies college and went on to serve in the Peregrine Falcon Special Forces Squad (PF Squad), starting in Metal Slug as First Lieutenant. As of Metal Slug 6, he is a Major. Marco is a gentle man but capable of falling into uncontrollable rages; the series' villain General Morden (who is responsible for the deaths of dozens of Marco's friends and colleagues in the PF Squad) will trigger these if he is even so much as mentioned.
Marco also appears in Neo Geo Battle Coliseum, being the first time he is seen in a fighting game. In the history of this game, Marco joins to Battle Coliseum tournament to stop once again General Morden, who's now financed by WAREZ Corporation to make new plans to conquest the world.