He is originally a commander of USN marine squad in a Taipei mission. During the siege of Taipei, his squad is eliminated by Changli Army and he's the only one that still alive. He went to USN ambassador in Taipei to investigate the possibility of his betrayal that causes the partial failure of the mission.
During the battle in USN ambassador, he used artillery tank and fight along side the Kazuki's team. After the battle, he joins the team.
In Alisa's storyline, Marcus is an enemy that fights Kazuki in the Futai Tunnel.
Outside of battle field, he is a father of twin girls and an alcoholic. In photo of his daughters wearing ninja suits sent by email, when it is processed by Kaleidoscope tool, it shows an UFO. It is not quiet sure what it means.