Private Carlos Santiago was a Gear soldier and Dominic Santiago's older brother, as well as Marcus Fenix's best friend and brother figure. He inspired both Marcus and Dom to join the Coalition of Ordered Governments army. Since he was a child, he was Marcus's closest friend, and later became his squad mate in the army. Carlos would go on to fight at the Battle of Aspho Fields, where he was killed in action. Carlos' death was the event that pushed Marcus over the edge, turning him even more emotionally distant from others. Dom mourned the death of his brother, but would learn how Carlos really died until he forced Bernie to tell him fourteen years after Emergence Day.

Carlos is not shown or mentioned in the games.