Frank is Bill's former partner in Lincoln. You learn about him in Bill's Town through the Note from Frank, and he also turns up earlier in The Outskirts as the contact named in a smuggler's note.
In The Outskirts, on the dead smuggler Ellie takes her flashlight from in the flooded subway, the Smuggler note names Frank as a trading contact who wants into the Boston QZ. Its margin shows the smuggler waiting at the Park Street exit and growing frightened as Frank never arrives.
In the High School Escape section of Bill's Town, the note lies on a table in the room nearest Frank's body. Frank tells Bill he hated him, wanted more than the town and Bill's stubborn ways could give, and admits he did get the battery Bill kept complaining about.
In the garage, you can start an optional conversation and hand Bill the note. He reads it, curses Frank and throws it down. Picking it up again counts as a separate artifact, the Note from Frank (crumpled).
Frank was bitten by a horde on his way back with the battery and hanged himself in his house rather than turn. Joel and Ellie find his body there, and Joel later offers Bill his condolences.
Bill
Bill is a paranoid survivalist and mechanic who lives alone in Lincoln, a town he has rigged with traps against the infected. He owes Joel favors from their smuggling deals, so in Bill's Town Joel asks him for a car, and Bill joins you to find the parts.
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.
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.
Ramirez
Ramirez is a FEDRA patrol soldier who catches Joel, Tess and Ellie just outside the Boston wall. The ambush happens in the Outside section of The Outskirts, and it is the moment Ellie's infection scan becomes a problem.