Marcus Fenix isn't all that interesting of a main character. His partner Dominic Santiago, on the other hand, is a lot easier to care about. He's complicated, with motives beyond "kill all thems thar Locusts and maybe find out about my daddy." See, Dom is looking for his wife, Maria, who went missing once the Imulsion (sic) hit the fan during Emergence Day. For the entire first game (and part of the second), Dom searches for information about Maria, getting glimmers of hope from time to time as he shows her picture to other survivors. And then... he finds her.
Say that Gears of War is for meatheads. Say that it's a dumb game with terrible writing. Say whatever you want - it doesn't matter. Epic nailed the moment when Dom finally finds Maria, a tortured, mindless husk of her former vibrant self, and we were bawling like we just got dumped the night before Spring Formal. Dom's voice actor, Carlos Ferro, actually stepped into the mo-cap suit for the first time to record this scene, helping to make this moment of despair even more human. And it was in Gears of War 2, so we were totally blindsided.