How does Miller use imagery and symbolism to characterize Larry, who never appears on stage but is so fundamental to the events and the people?...

Miller uses imagery and symbolism to characterize Larry is
crucial to the play even though Larry never appears on stage.  The major symbol that
revolves around Larry is the apple tree in the Kellers' yard.  The tree was planted as a
memorial to Larry after he disappeared in the war, and at the opening of the play, the
characters realize that the tree has been split in half by lightening during the night. 
Ironically, the time is August, the month of Larry's birthday.  Symbolically, the
splitting of the tree foreshadows the split that will occur later between the family
members as they grapple with the issue of Larry's disappearance as opposed to his
death/suicide--Kate will hold her belief in Larry's disappearance for as long as she can
while other members of the family come to accept that Larry is
dead.

Miller also uses imagery to characerize Larry.  In
the letter that Larry has written and sent to Ann, he describes his experiences in the
war so that Ann (and readers) can witness what he sees every day.  By the end of the
letter, the reader understands the horrors of war that have had such a great effect on
Larry.  Larry's view of war is a direct challenge to Joe Keller's decision to ship the
faulty machine parts and keep it a secret, making Larry's presence crucial to the
development of plot and theme.