biff ?

The source of Biff Loman's resentment of his father is the betrayal he perceives Willy to have committed by cheating on Linda. Up until the point where Biff finds Willy with the woman he believes Willy is an amazing, genuine and impressive character, hence why Biff asks Willy to talk to his maths teacher, he thinks Willy will be able to change his teachers mind. However, discovering his father with an another woman quickly shatters any respect he had for him..."he's a fake!"