Dominic "Dom" Santiago, Marcus Fenix's friend and fellow CoG soldier is of my favorite characters in recent years. Dom has saved Marcus on two occasions that we know of, once saving him from the death penalty after testifying in his trial, and again at the very beginning of Gears 1 rescuing him from Jacinto Maximum Security Prison which has been overtaken by the Locust Horde, and accompanies Marcus through all of Gears 1 as a major asset to complete their Light Mass Bomb deployment mission. But Dom is not just a supporting character, Dom's story is deep enough to make him a main character.
Though humanities plight against the Locust Horde is global struggle for all humans on Sera, the war with the Locust Horde has a more personal meaning for Dom due to the disappearance of his wife Maria on Emergence Day. The struggle to find his wife while fulfilling his duty as a Gear has Dom doing things most Gears wouldn't, like associating with Stranded in order to find information on Maria's whereabouts. Dom's search, however bleak it may seem, keeps him going and his refusal to give up, shines a small ray of optimism and hope in this story of a war torn world.
A note worthy point is Dom's status within the CoG army. Though he has served for years during the Pendulum Wars, including the Aspho Fields Battle (as mentioned by Carmine) and the ongoing Locust-Human War, Dom still has a rank of Private. This could be due to his testifying in favor of Marcus during his desertion trial, which could have possibly earned him mistrust among the Gears due to Marcus' reputation.
Another thing about Dom that is something I have thought about and talked about with others, is the fact that humanity was basically at a stand still with the Locust Horde, that is until Marcus Fenix is broken out of prison, by Dom, and leads Delta Squad against seemingly never ending Locust and insurmountable odds to deploy the Light Mass Bomb, a Major victory against the Locust. If Marcus is a major factor in the human victory against the Locust in the Gears of War story, then by association (and the fact that he is part of Delta Squad) Dom is also responsible for saving humanity from the Locust Horde.
Dom at first seems like a simple character, just another soldier who is friends with protagonist Marcus Fenix, but as you get to know him and his story, Dom becomes a more complex character who's story and past rivals that of Marcus' (desertion, his father and such). With his Maria tattoo proudly displayed on his right arm and his lancer in hand Dom continued the fight against the Locust and the search for his missing wife, and I hope his story ends well and does not end early.
I still need to play Gears 1, but from what I hear, everyone's story is going to get expanded on much more in Gears 2, with much more emphasis on characterization, so I'm sure you'll get to see a lot more of Dom.
I hated Dom, the guy was always asking me to come revive him, and i was all like, "Dude, just chill I'm shooting shit, leave me alone".
Good plot device? Yes. Reliable AI partner? No.
In before GoW had a story?
Anyways. I agree with Jayrock LOL