Jeremy is a FEDRA soldier in Pittsburgh, mentioned in the Lost Hill note you can find in the Pittsburgh chapter. A squadmate writes that Jeremy and a soldier named Pam were left behind when the hunters took Troy Hill.
After you jump over the bus into the Pittsburgh quarantine zone, check the booth on the far left for this note in a blue FEDRA notebook. Its writer says rebels stole a mortar launcher and overran Troy Hill, that with the Strip District they now hold the whole river, and that barely half the squad survived the retreat. Jeremy and Pam were stranded on the far side of the river, and the writer prays they are still alive.
The note is the last word on him; whether he survived is never answered.
You read it in Alone and Forsaken, after the ambush that rams Joel's truck into a hardware store and the walk through the hunters' hideout. The booth stands at a military checkpoint that FEDRA lost long ago, and notes like this one are how Joel and Ellie piece together that the city's own people threw the soldiers out.
Henry
Henry is a young survivor from Hartford who is trying to get his little brother Sam out of hunter-held Pittsburgh. He and Joel get off to a bad start in a brawl, then team up to escape the city, and he fights beside you through the end of Pittsburgh and The Suburbs.
Mastros
Captain Mastros is the senior FEDRA officer left in the Pittsburgh quarantine zone. He appears only in two artifacts from the Pittsburgh chapter: the Abandon zone note he wrote and the Mob attack note about him.
Sam
Sam is Henry's 13-year-old brother, a quiet, anxious kid who becomes Ellie's first friend her own age on the road. You travel with him from the hideout in Pittsburgh through the sewers and suburbs in The Suburbs.
Ellie Williams
Ellie Williams is the 14-year-old girl Joel is hired to smuggle out of Boston, and your companion for most of The Last of Us Part I. She was bitten weeks earlier and never turned, so the Fireflies believe her immunity could lead to a vaccine. You play as her in the winter chapter and in the Left Behind DLC.