Tommy is Joel's younger brother in The Last of Us. He helps Joel survive through the initial outbreak in their town in Austin, Texas, already showing competence in handling firearms by then.
Sometime in between the game's prologue and the 20 year fast-forward into the post-apocalypse, he and Joel had a falling out, splitting the two apart until they meet again in the Fall chapter of the game.
It was Joel's descent into ruthless survival mentality that led to the brothers' hostile separation. Tommy found purpose in the government resistance group the Fireflies, but was soon disenchanted with the organization as well and left.
He manages to start a community in an abandoned hydroelectric power plant with the help of his wife Maria. Together, they lead their small society to keep things secure and civilized. His one dream goal is to restart the nearby dam to bring back electricity and its comforts to their little corner of this god-forsaken world.