Willy is the salesman that dies, so you can pretty much assume before you start that he plays the central role, which of course he does. Overall, Death of a Salesman centers on Willy’s struggle to find self-validation amidst his unrealistic dreams. His personality and needs drive many of the play’s interactions, and are also a primary cause of the other characters’ suffering.
Biff might not traditionally be considered a protagonist since the majority of the storyline is driven by Willy. However, Biff is a central character and the only one who shows personal development in the play, bringing him to the forefront. In addition, Willy tries to pass the torch on to his son as he commits suicide, again bringing Biff to center stage, so to speak.