Apart from the character profiles given above (and good ones, too!), Tom is the pivotal character in the story, not Laura. He is the person in primary conflict, and the story line revolves around his choices and their eventual consequences. Both Amanda and Laura, as endearing as they may be, are flat characters and catalysts, bringing out both the best and the worst in Tom. They create a situational context for the conflict in Tom to work itself out (though not altogether resolved).
In a parallel way, Jim does the same thing for Laura in "the serious conversation" scene when the unicorn is broken. Spectators get the feeling Laura has changed, has taken an initial step forward, even if the outcome of this change isn't clearly seen in the rest of the play.
Tom reveals his dual role in the opening scene of the play: "I am the narrator of the play, and also a character in it." Tennessee Willaims uses Tom as a narrator to address his audience directly and inform them about the background of the action of the play and the relationships of the characters and their different roles. As a character, Tom is Amanda's son and Laura's brother. After his father has abandoned the family Tom becomes the sole breadwinner of the family. Consequently, he feels cheated that he has to sacrifice his youthful pleasures to fulfill his domestic responsibilities and he is constantly at loggerheads with his mother on this issue. Finally, after his botched attempt at getting "a gentleman caller" for his physically challenged sister he like his father runs away from home.
Amanda, is Tom's and Laura's mother. Her husband has deserted the family, consequently throughout the play she is stressed out at having to make both ends meet. Her only aim in life is "success and happiness for my precious children!" (sc.5). She does her utmost to keep Tom the only source of financial support under her control but doesn't succeed and she is left in the darkness at the end of the play.
Laura is the physically challenged girl in the family. She limps and because of this she suffers from an inferiority complex. Tom arranges for Jim "a gentelman caller" to visit her who cures her of it, but doesn't marry her.