He is known for taking out many Locusts by himself, and is not easy to take down. Marcus and Delta-One had to neutralize a force of Locust raiders in order to protect the encampment, in what became known as the Battle for Fucked. As he boards the Grindlift, the drilling device, Skorge, the Kantus priest leader, leaps onto the Derrick and engages in a chainsaw duel with Tai and Dizzy.
Marcus wouldn't wear his COG issued helmet but his "do-rag" was acceptable headgear as long as the ties were tucked neatly.
Like the rest of humankind, he saw everything change on Emergence Day. Brand did not have tactical command over Delta. The newly formed Delta Squad is forced to retreat from battle into the Tomb of The Unknowns, and during said retreat the current Lieutenant for Delta, Minh Young Kim, is killed by General RAAM.
Later on, Private Anthony Carmine is shot in the head by a sniper while searching for Alpha Squad.
Marcus and the rest of Delta eventually locate one member of Alpha Squad, Augustus Cole. Delta Squad is quickly deployed into the field on a mission to locate the missing COG unit, Alpha Squad.
During a battle ten years after E-Day, Marcus abandoned his post in a vain attempt to save his father, Adam Fenix. At this point and further on, Dizzy is still MIA.
Leading Delta since Sgt.
He was sentenced for life in Jacinto Maximum Security Prison, where he was prisoner number 098356-GX. The Brumak is infected with Imulsion after having to step in it clear out the aforementioned underground space, and turns into an explosive, Lambent Brumak. He soon dies, and the remaining four chainsaw themselves out of the worm after disabling its three hearts. Part of his cold demeanor stems from the loss of his best friend Carlos Santiago and a strained parental relationship during his childhood and adolescent years although, as Maria Santiago points out, his father at the very least cared deeply about him but apparently didn't know how to truly show it. After scouting the area, Marcus along Delta Squad, found a underground tunnel complex, which they explored and found ammunition and rations. Working alongside his son, James Dominic Fenix, and his two allies, Kait Diaz and Delmont Walker, he brings the Swarm to the new Coalition of Ordered Governments' attention and later assists Kait in discovering her past. Three years later, Marcus' mother mysteriously disappeared, leaving Marcus with a severe traumatic effect.
He is able to show sorrow when losing a teammate. Dom comes and frees Marcus himself. Four months after the city of Jilane fell to the Horde, a city infamous for artificial insemination and other ways for the COG to boost human populace, both by traditions and by science.
Marcus is reluctant to follow him on the search, but gives in. Disillusioned with the New COG, the only thing greater than his bravery in battle is his loyalty to family and friends. They find Maria, who is bruised up and battered, similar to Tai, and realizing that she will never function again, Dom kills her so she won't have to suffer a slow death. At that battle all of the 26th officers where killed except for Mataki, Marcus took over an UIR Asp and stopped the UIR army advance on the Aspho Fields bridge and saved the commandos escaping from Aspho Point from enemy helicopters but was not able whole to save his best friend Carlos Santiago who commited suicide by grenade after being wounded (so Marcus wouldn't come and try to save him) on a bridge after attempting to destroy an Indie Asp.
The fate of the pilot is unknown, but the squad wakes up inside the worm, finding that Carmine is missing. After Gil was killed he headed for The Rusty Nail which is the only place to find him after a mission. Though Marcus was struggling with his personal feelings with Anya, she revealed that she shares Marcus' attraction.
Baird and Cole reunite with Marcus and Dom, so the four of them enter the Locust palace and face the queen. Marcus orders the Hammer of Dawn to attack, but Baird has to blind-fire it because he doesn't have full control.