In A Doll's House by Henrik Ibsen, Nora will ultimately discover that her life as Torvald's "little squirrel" is shallow and that her marriage will not withstand the dramatic turn of events. Christine Linde is the ideal contrast to Nora, exposing Nora's dependence and her one-dimensional relationship with Torvald Helmer, her husband.
Torvald treats Nora like a child, even trying to appease her disappointment when she suggests that they spend a little more money and be "more reckless" due to Torvald's new job. Torvald's response is to patronize her further when he says, "One would hardly believe how expensive such little persons are," even suggesting that she has inherited her father's propensity for spending, and cannot help herself.
When Christine Linde, an apparent "stranger" arrives Nora does not immediately recognize her and this serves to emphasize how far removed Nora is from the life that Mrs Linde has lived. Nora's self-absorption is apparent as she talks excitedly and Nora's astonishment that Mrs Linde's husband left her penniless and childless is beyond Nora's comprehension. Mrs Linde's arrival therefore helps to develop Nora's character and forewarn the audience / reader that Nora's whole existence is based on her warped understanding of the roles of husband and wife which will only serve to create an untenable situation but one that, thanks to Mrs Linde, Nora can learn from.
Mrs Linde's arrival also develops the plot as it is she who decides that it is time for Nora to tell the truth. Her arrival foreshadows other unexpected events and occurrences which will change the nature of Torvald and Nora's relationship.