Linda functions as a surrogate for the audience in this play, offering ways to sympathize with both Willy and Biff. She can be said also to represent a conscience in the play and to function as a stand-in for a formal chorus.
The men in the family form a three-part character structure with Happy and Biff both serving as explorations of Willy's nature. Biff and Happy are very similar to their father, but represent different paths that Willy could have taken regarding his deepest issues and problems.
Willy's central problem can be seen as his tenacious attachment to a false dream. Biff and Happy both share this tendency for a while, until Biff chooses to let go of the fantasy and embrace reality. This is when he stops pretending that he is better than he is. This is also when he becomes his own person, ceasing to blame Willy for his failures.
Happy does not take this step and shows real moral weakness. He promises to carry on Willy's dream, Willy's vision of the world, and, in this way, to continue living in the dream that Biff has now rejected.
Taken as a whole, we can look at these three characters as examples of the "average man" in his potential, facing his limited options, and struggling to come to terms with the emotional reality of mediocrity in America.