In the beginning of the play, the audience is first exposed to Nora, her behavior, and the dynamics of her marriage. By all accounts, Nora embodies happiness, joie de vivre, and bliss. This exposure leads to the belief that Nora's life is all put together and that she has no worries in the world.
Shortly after the beginning, in comes the character of Mrs. Linde. From the start, it is clear that Mrs. Linde is the complete opposite of Nora. Her voice is described as "dejected" and "timid." When Nora sees her, she immediately notices that Mrs. Linde has changed greatly, not necessarily for the better. Then, we hear the story of Mrs. Linde: she is alone in the world, needs employment, and longs for companionship.
When juxtaposed in the same entry scene, it is clear that Ibsen intended for Linde to be the polar opposite of Nora. The audience will form a contrasting view of the two women that will help them to define who Nora is. That is the way that Linde's presence helps to define Nora's character at the beginning of the play.
Still, another big juxtaposition will come later on in Act I, when we will witness how Linde's presence not only helps to define Nora's traits but also challenges Nora's true self to come out. It is here that Nora decides to tell her secret to Christine as a way to show that she (Nora), too, has gone through vicissitudes and sacrifices on behalf of others. It will be through this confession that Nora's character will be further delineated, and the audience will realize the kind of person that Nora is: a woman that has been socially groomed to play an expected role for which she is neither appreciated nor valued—a role which she will end up resenting.
I was your little skylark, your doll, which you would in future treat with doubly gentle care, because it was so brittle and fragile. . . . It dawned upon me that for eight years I had been living here with a strange man, and had borne him three children—. Oh, I can't bear to think of it! I could tear myself into little bits!