The concept of a "tragic hero" derives from Aristotle's understanding of Greek tragedy. Ibsen's A Doll's House is not a Greek tragedy, and thus, it does not really follow the pattern discussed by Aristotle. While Nora is the protagonist of the play, she is not a "tragic hero."
The concept of "hamartia" is a Greek term mean an "error", deriving from the Greek verb "hamartein" meaning to err or miss, specifically in the sense of an arrow missing its target. In tragedy, this means an act that sets a character on the wrong path, and which, like an arrow shot from a bow, cannot be taken back.
In the case of Nora, one could argue that forging her father's signature was precisely this sort of irrevocable act that sets events on a course that inevitably lead to disaster. However, the ending of the play isn't really tragic. Nora's new understanding of herself and her life lead her to leave her marriage, but most viewers see this as a positive step rather than something that evokes "fear and pity."
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