Jerry Anderson is the lead doctor of the Fireflies at St. Mary's Hospital in Salt Lake City, the man in charge of their research into a cure. You only meet him near the end of the game, in the chapter The Firefly Lab, and his part in the story is best discovered by playing it.
In the hospital you can find a recording Jerry made before operating on Ellie. He goes over the infected people the Fireflies examined before her, sets out how her case differs, and says he hopes she is the breakthrough humanity has been waiting for.
Jerry is the surgeon preparing to remove the infection from Ellie's brain, an operation she will not survive. When Joel bursts into the operating room, Jerry stands in his way with a scalpel and tells him how many lives Ellie could save. Joel kills him and carries Ellie out of the operating room.
Ethan
Ethan is a Firefly soldier at Saint Mary's Hospital in Salt Lake City. In The Firefly Lab he finds Joel and Ellie after they arrive, and Marlene later puts him in charge of walking Joel out of the building.
Kari
Kari is a Firefly surgical assistant at Saint Mary's Hospital in Salt Lake City. You meet her in the operating room at the end of The Firefly Lab, where she is helping the surgeon Jerry Anderson.
Marlene
Marlene is the leader of the Fireflies, the militia fighting the military in the quarantine zones. Wounded in Boston, she hires Joel and Tess to smuggle Ellie out of the city in The Quarantine Zone, and you meet her again at the Fireflies' hospital in Salt Lake City near the end of the game.
Anna
Anna is Ellie's mother, known only through a letter Ellie carries and a few remarks from people who knew her. You can read her letter, the Note from mom artifact, from Ellie's backpack at the start of Lakeside Resort.