Biff is a mirror image of Willy himself, and through the play, comes to know and understand his father. Betrayed as a young adult by Willy's adultery, Biff lets go of his dreams and wanders about, disillusioned, and still searching for the popularity and wealth that Willy encouraged him to desire at a young age. Biff has realized that his father's desires had little to do with "success", however. He sees that his father's perception of the world has become tainted, and even shout's to Willy that he (Biff) is "nothing". Biff also understands that it is this skewed perception, the idealization and search for an illusory American Dream, that kills Willy in the end. Biff relinquishes his father's goals at the end, opting to go back west and live a quiet life on the ranches he enjoys, rather than pursue material wealth. Happy, on the other hand, is caught up in Willy's dreams, and vows to carry them on at Willy's death, never understanding that the false pursuit of wealth and happiness is what killed him.
Biff is certainly Willy's favorite. Biff is the strong picture of an American male. He is popular and athletic at an early age. Willy, weak spirited and unconfident, puts Biff on a pedastal, and believes Biff will deliver Willy's American Dream of wealth. Willy brags relentlessly to Bernard in this regard, and when Biff fails out of school, still spends the next 15 to 20 years pressuring his son to pursue a mainstream existence in American business. Happy, too much like Willy (womanizing, eager for the limelight, suspectible to popularity), is thus a failure.