A no-nonsense commander, Marcus commands brethren under his command with a steady hand and complete intolerance for failure. As he says, "You do not want to know the consequences of failure". Kane himself has raised and taught Marcus.
A Black Hand zealot, once he caught wind of Kane's plan to ally himself with GDI, he became enraged. He betrayed the Prophet and gathered an army to attack GDI forces. With approval from GDI leadership with whom Kane was negotiating behind the scenes, a GDI commander was dispatched to apprehend Marcus and bring him in.
As the player's commander, Marcus gives out orders and guides him through the first few missions.