Which character change/progresses the most in Death of a Salesman? What types of changes occur to this character? What is some evidence to prove...

I would say that Biff changes the most in the play as he
realises that he has control over not only his own success and failure, but his
father's.

Biff was seen as the hero, with prospects,
sporting prowess and good looks. He also possessed some of his father's rose-tinted
optimism in teh early days. Biff however clearly gave up on striving for success and
recognition after he saw his father with the other woman. From that point he sees his
father as a 'fake' and has avoided dealing with the crushing disappointment of his
father's fallibility. Once he is made aware of the seriousness of Willy's mental state,
he steps up to support his parents-

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BIFF: ...I've been remiss. I know that, Mom. But
now I'll stay, and I swear to you, I'll apply
myself.

Unfortunately Biff is
unable to get the truth about both of their failures through to his father, and though
he tries to comfort his mother, Willy is beyond help in terms of seeing
reality-

BIFF:
I am not a leader of men, Willy, and neither are
you.

Willy's last thoughts
are of Biff scoring a touch down for him as he promised. Willy's suicide can be seen as
prompted by him finally accepting what Biff tried to tell him all
along.