Who is Lieutenant Billy Prior from The Eye in the Door and what is their importance?

Billy Prior is a young working-class man from Salford, England. An only child, Billy was raised in an abusive home by a violent and brutish father and a passive mother. Early on, Billy distinguishes himself because of his intelligence and affability. Billy is the local boy who "made good" when he received an officer's commission. In more ways than one, Billy Prior is someone who lives two distinct lives. Everything about Billy Prior is paradoxical. He is an officer from the working class, a man who develops a second self in order to deal with the trauma of his childhood and the war. Billy is by turns sadistic and tender. Although he has a steady girlfriend, Sarah Lumb, Billy Prior is bisexual.