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.
Ethan is on patrol with another Firefly when he finds Joel trying to revive a nearly drowned Ellie. Joel is too focused on her to raise his hands, so Ethan clubs him unconscious.
Marlene tells Ethan to march Joel out of the hospital and shoot him if he tries to stop the surgery. Ethan clearly dislikes Joel and seems to hope for an excuse.
In the ward Joel spots his backpack, knocks Ethan's pistol up as it fires, beats him, and shoots him in the stomach to learn where Ellie is. Once Ethan tells him, Joel kills him. Other Fireflies later find the body and raise the alarm. You can still pick up ammo from his corpse when play resumes.
Jerry Anderson
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.
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.