A Streetcar Named Desire is a tragic drama. Tragedies involve a character (usually from a high social position) experiencing a reversal of fortune, leading to their downfall. This character is also brought low by a tragic flaw.
Blanche Dubois is the tragic heroine of the story. Coming from a life of high social class and romance, Blanche's life is besieged with almost every kind of loss even before the proper start of the play: she loses her family, her estate, her youthful dreams, and her young husband. During the play, Blanche tries to get Mitch to marry her as a last resort, hoping she can find a new start through the relationship. Unfortunately, she cannot outrun her past: Mitch learns the truth and abandons her, while Stanley rapes her, triggering a final mental breakdown. By the last scene, Blanche has lost her hold on reality and is confined to an asylum.
Blanche's tragic flaw is her belief that she can fight reality and win. She lies all the time, not from malice but from a desperate desire to make her lies appear true, whether she's fibbing about her age or claiming she does not have a drinking problem. Stanley becomes a stand-in for reality itself, ultimately pushing Blanche to the point of no return.