The two characters who I immediately think of in response to this question are Linda and Biff. This is because both in the play show their true strength as a result of the tragedy that they face in this play.
Linda is of course a character who exhibits her strength throughout the entire play. Her situation is unenviable, as the man she loves is slowly losing his mind and can be quite aggressive and shouts at her all the time. She has to cope with his moods and his ups and his downs and to bear the constant worry that he might kill himself or die in an accident. She also has to bear with this problem by herself because of the way in which her two sons are largely unhelpful and rather selfishly preoccupied with their own lives.
However, the other character who I feel shows significant strength of personality is Biff. This is because, during the course of the play, he confronts who he is and the failure that is his life and tries to acknowledge this before his father. He at least tries to challenge his father's thoughts in this area:
Today I realised something about myself and I tried to explain it to you and I--I think I'm just not smart enough to make any sense out of it for you.
He confronts both himself and Willy with their self-deception, and at the end of the play he clearly has managed to accept who he is and reject who he isn't, as is shown when he says to his deluded brother, "I know who I am, kid." This is strength indeed.