Tommy Miller is Joel's younger brother, a former Firefly who now helps run the settlement at Jackson, Wyoming. He drives Joel and Sarah through the chaos of the prologue, and twenty years later Joel tracks him down at a hydroelectric dam in the chapter Tommy's Dam, hoping he will take Ellie to the Fireflies.
Tommy calls Joel repeatedly the evening the infection breaks out, then picks up Joel and Sarah in his car and tries to reach the military blockade. When a crash wrecks the car he takes Joel's revolver to cover them, holds back infected at a bar so the others can run for the highway, and catches up just in time to shoot the soldier who has Joel in his sights.
In the years after the outbreak the brothers make their way to the Boston quarantine zone. Tommy is haunted by what they did to survive and decides to join the Fireflies, which leads to a bitter fight with Joel. He tells Joel he never wants to see him again and walks away.
In the fall of 2033 Joel and Ellie reach the Jackson County power plant, where Tommy and Maria are getting a turbine running for the town. Tommy welcomes his brother, shows him the generator start up, and offers him the old photo, which Joel cannot bring himself to take. When Joel asks him to take over the job of delivering Ellie, Tommy refuses, saying his family is his cause now. Joel shoves him against a locker just as bandits hit the dam.
After the attack Tommy agrees to take Ellie after all, which upsets Maria. When Ellie runs off on a horse, Tommy and Joel chase her through the woods, fight bandits at some cabins, and find her at an abandoned ranch house. Tommy keeps watch downstairs while the other two talk, then warns them when more bandits arrive. Back at the town gates he tells Joel the Fireflies' lab is at the University of Eastern Colorado and promises there will always be a place for him in Jackson.
Tommy's shot saves Joel on the highway, but Sarah has already been hit and dies in front of him. He also kept nightmares from the years he and Joel spent as hunters before they reached Boston.
Joel Miller
Joel Miller is the man you play for almost all of The Last of Us Part I. A hardened smuggler in the Boston quarantine zone, he takes a job escorting 14-year-old Ellie out of the city, and the drop-off turns into a year-long trip across the country to find the Fireflies.
Buckley
Buckley is the dog at Tommy's hydroelectric dam in Jackson County. You meet him in Tommy's Dam outside the generator building as Tommy walks Joel to his office, and Joel can stop to pet him.
Maria Miller
Maria Miller is Tommy's wife and the leader of the settlement in Jackson, Wyoming. You meet her at the hydroelectric dam in Tommy's Dam, where she first holds Joel and Ellie at gunpoint before learning Joel is her brother-in-law.
Seth
Seth is a Jackson resident on guard duty at the hydroelectric dam in Tommy's Dam. He is the man beside Tommy on the wall, rifle trained on Joel and Ellie, when they first reach the gate.