Marco is one of the riflemen in the assault team, the other being his Commander, the player, and another operative. They are holding M72 LAW rocket launchers on standby. He then holds his position to eliminate the resistance. After the firefight, the Commander orders Marco to stay put, holding on to keep the enemy pinned down.
The Assault team:
Karl (the other operative)
Marco cannot be killed, even if the player shoots him or does not assist Marco to kill enemies. He appears to have unlimited health although he will still be stunned by gunfire.
Marco's appearance was only in the firefight.
He and the Commander are equipped with M4A1 rifles.
The other member with an MP5 is not him as he is equipped with an M4A1 rifle.
Despite the commander ordering Marco to stay down, he cannot be found when the player is at the streets. He may have left alongside with the commander, though it is likely the developer team forgot to include him in a later encounter.