Coalition of Ordered Government (C.O.G.)
Marcus Fenix
The main character, a soldier thrown in prision 14 years prior to the start of the game. Before "Emergence Day" (the day that the Locust Hoarde invaded), there was a huge war going on between all of the nations in the world. When oil supplies were running low, an alternate energy source, called "Immulsion," was discovered. It turned out that Immulsion was not only more productive than oil, but it was environmentally friendly. Some nations had bountiful amounts of Immulsion, some had none. When the less lucky nations attacked the ones with all the extra energy, the Pedulum Wars had begun. Sera (the planet Gears Of War takes place on), was torn apart in war, and no one saw the Locust invasion coming. When the Locust attacked, Marcus was instructed to protect civilians and to fight the Hoarde, but instead he went to save his father and was charged with treason.
Dominic Santiago
Dom is Marcus' best friend and comrade, this is exaggerated by the fact that in co-op play the second player is Dom. When Marcus was on trial he stood by testifying that Marcus was not a coward and saving him from the death penalty. His wife Maria disappeared at the onset of the war so he is fighting the locust for her sake. He is also owed some kind of debt by Franklin, a Stranded, that is paid back during the campaign.
Augustus Cole
Augustus Cole, aka "The Cole Train", is a former professional Thrashball player for the Cougars. He craves action and prefers the most direct path to his next fight. What he lacks in finesse, he makes up for in raw energy. Along with his child-like enthusiasm and charm, Cole is confident in his own abilities to overcome anything. He has yet to be proven wrong.
Damon Baird
Baird is the cynical and often argumentative tech expert of the squad. He holds a grudge against Marcus for getting promoted to team leader instead of him. He's revealed to be the tech expert when he is assigned to repair an APC in act 4.
Lieutenant Minh Young Kim
Lt. Kim is a "by the book" soldier, following every single rule and code of conduct to all things COG, making him an excellent Lieutenant. When the squad reaches the ruined courtroom(where Marcus was condemmed to prison), Marcus makes a crack about the court, labeling it for it's "lies". Kim then barks at him for his disrespect. At the end of Act 1, a King Raven helicopter arriving to evacuate Alpha and Delta Squad is taken down by Nemacyst spores, and Kim is separated from the rest of the Gears in the ensuing firefight. Amidst all the chaos, Kim orders a retreat and distracts the incoming Locust Horde, unaware of General RAAM approaching him from behind. Too late to defend himself, RAAM impales Kim on a jagged sword.
Private Carmine
"Voted most likely to get shot" in the COG Academy (according to his multiplayer character description), Pvt. Carmine is a member of Delta Squad at the beginning of the game, along with Lt. Kim, Marcus Fenix and Dominic Santiago. In battle Carmine will often rush out into the open, and has a gung-ho attitude. In Act 1, while standing up to complain about his Lancer rifle jamming, he is shot in the head by a Locust sniper. His face is never revealed and his character is underdeveloped resulting in him being a kind of throw-away character that is quickly forgotten once he dies.
Lieutenant Anya Stroud
Anya is the only female character in Gears of War and only briefly appears in person. She is the Gears' source of information constantly informing them of incoming enemies and satellite activity. Without Anya the Gears would undoubtedly have died.
Colonel Victor Hoffman
Like Anya Stroud, Hoffman is rarely seen in Gears of War, often sending orders via radio. Hoffman first appears in the third cinematic, telling Marcus he is a traitor and doesn't deserve to wear the uniform, and the final cinematic aboard the King Raven with Dom, Cole and Baird after defeating General RAAM. Initial interactions between Marcus and Hoffman are extremely confrontational, although Hoffman comes to have a grudging respect for Marcus by the end of the game; in the final cutscene, as Marcus leaps to a helicopter from a train which is plumetting into a canyon below, Hoffman reaches his hand out to help Marcus onto the helicopter and gives a small, yet noticable smile of approval. In the final cutscene of the game, he is heard giving a speech proclaiming victory over the Locust Horde and praising the Gears.
"Jack"
A Jack of all Trades robot, Jack for short. It has limited A.I., does not talk, and responds only to simple commands. It is always alongside Delta Squad, hovering about with a cloaking device that makes the robot unseen until it decloaks to make itself useful.
Jack is used for cutting doors, communications, first aid and minor field surgery, as well as other delicate operations both during and outside of combat. Often the Gears will have to defend Jack while it cuts open doors. In Act 4 of Gears of War, Jack is also used to download and store Adam Fenix's research data, which contained amazingly detailed maps of the Locust underground systems.