If we were going to refer to A Doll's House as a tragedy and to analyze the characters for their misinterpretation as their hamartia tragic flaw, we could actually refer to either Nora or Torvald as the tragic hero. Nora can more likely be considered the tragic hero because she is really the protagonist of the story. Furthermore, her misinterpretation of her husband and her naivete about society and law have led to dramatic changes in her life. However, at the end of the play, Nora seems quite content to begin life over again. She seems quite content to leave both her husband and her children and to begin educating herself and striving to become a "reasonable person" (III). While Torvald is not the protagonist, he is the only character who feels he has suffered a devastating blow. Therefore, let's analyze Torvald as the tragic hero.
Torvald's tragic flaw is that he accepted society's characteristic treatment of women as the way in which he ought to behave. Moreover, he misinterpreted his wife, including her desires and her motives. He only saw her as a ridiculous person, rather than the strong, self-sacrificial woman she truly was. As Nora says to Torvald in the final act, "You don't understand me, and I have never understood you either" (III). Nora had expected that when Torvald learned of her forgery he would instantly recognize it as having been an act of love and self-sacrifice in order to save his life. She further expected that he would be self-sacrificial in return and take all the blame for her actions upon himself, which would ruin his reputation. Torvald intimated earlier that he would sacrifice himself for her if ever they were put in any danger, such as Krogstad trying to slander Torvald's reputation, when Torvald says, "Come what will, you may be sure I shall have both courage and strength if they be needed. You will see I am man enough to take everything upon myself" (II). However, after Torvald reads Krogstad's letter his first reaction is to blame her for destroying his reputation, demanding of her, "Do you understand what you have done?," even going so far as to call her "a hypocrite, a liar--worse, worse--a criminal!"(III).
Nora doesn't stand for this treatment. Torvald soon realizes that all these years he has thought of her and treated her as someone to be trifled with. Now she is leaving and he is genuinely brokenhearted. We see just how truly devastated Torvald is in Ibsen's final lines, including the stage directions:
[sinks down on a chair at the door and buries his face in his hands.] Nora! Nora! [Looks round, and rises.] Empty. She is gone. (III).
While Nora suffers losses too, such as clothing, lifestyle, and children, her losses are all self-inflicted. Torvald is the only one who has suffered any real tragic loss. Therefore, it can be said that Torvald is the tragic hero, that his ill-judgement of his wife is his tragic misinterpretation, and that the influences of society that he accepted are his tragic flaw.