Delta Squad changes quite a lot during Act 1: Ashes, but remains the same for the rest of the campaign.
The commander of Delta Squad during Act 1. He has a grudge against Marcus at the start, but comes to have a respect for him.
Private/Sergeant Marcus Fenix
Marcus Fenix was broken out of Jacinto Penitentiary by Dominic Santiago, 14 years after Emergence Day. He joins Delta Squad and helps to find Alpha Squad and the Resonator. When Lt. Kim is killed by RAAM at the end of Act 1, Marcus is promoted to Sergeant and becomes commander of Delta Squad.
Private Dominic Santiago
Carmine is a cocky young soldier always raring for a fight. He is the only Delta Squad member to wear a helmet which restricted his peripheral vision, so he didn't see the Locust sniper that killed him. He is the first Delta soldier to die.
The "Cole Train" is a former "Thrashball" professional who joined Alpha Squad after Emergence Day. He is found by Delta taking on an entire Locust squad... and winning. He joins Delta Squad, replacing the deceased Carmine.
Baird is a close friend of Cole, and another member of Alpha. He is very cocky, but cares for his fellow soldiers. He is reluctant to follow Marcus Fenix. When Lt. Kim dies, Baird joins Delta in his place.
Rojas is a soldier only seen once in the campaign... as a horribly maimed corpse. Little is known about him, except he has a two-year-old son.
Colonel Hoffman isn't actually a member of Delta Squad, but is a friend to all of them, even Marcus in the end. When the Lightmass Bomb is detonated, Hoffman pulls Marcus on board the Raven helicopter, with a reluctant smile on his face.