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.
When Joel and Ellie try to open the plant's gates, Seth and Tommy appear on top of the wall with rifles. Tommy recognizes his brother and lets them in, and Seth goes back to watching the main gate.
When bandits raid the dam later in the chapter, Seth is among the defenders who help hold it.
A brick guardhouse stands in the corner just before the plant gate, and the Power plant map hangs on its wall. Grab it before you try the gate, because that is when Seth and Tommy appear on the wall above you.
The plant supplies electricity to the settlement Maria and her father run. While Seth keeps watch, Tommy shows Joel around and the workers bring the generator back online.
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.
Tommy Miller
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.
Callus
Callus is the horse Joel and Ellie ride from Jackson County to the University of Eastern Colorado and beyond. He first carries Joel when Ellie runs off in Tommy's Dam, and Ellie names him on the road to the university.