At first our protagonist, Nora, seems like a bit of a ditz. When her husband, Torvald, calls her things like his "little squirrel," his "little lark," and, worst of all, a "featherhead" (ouch), she doesn't seem to mind (1.5-1.16). In fact she seems to enjoy and even play into it.
When Torvald first calls her a spendthrift, we're inclined to agree. In her first moments onstage, we see her give the porter an overly generous tip, come in with tons of Christmas presents, and shrug at the idea of incurring debt. Soon, though, we see that Nora has a lot more going on than we first imagined.
When Nora's old friend Christine arrives, Nora divulges a little secret. She's not just leeching off her husband. On the contrary, she saved his life... by getting them both into massive debt.
Unbeknown to Torvald, Nora borrowed money so that they could afford a year-long trip to Italy. Doctors said that Torvald would die without it—but that he shouldn't know how bad his condition was. Those Victorian doctors, eh? "We prescribe a yearlong holiday... but don't tell the patient that the holiday is a holiday. Yeah, we don't know how you'll do that either. So long, sucker."
Rather than being the spendthrift that both Torvald and Christine accuse her of, Nora's actually pretty dang thrifty. She's been secretly working odd jobs and even skimming money from her allowance to pay back the debt. Later on we learn that Nora was so determined to save her husband that she committed fraud to do so. This choice shows that Nora is both daring and tenacious. She values love over the law. When her secret is revealed we know that, beneath the ditzy character she plays for her husband, there's a whole other (highly competent) Nora waiting to come out.
This other, more capable Nora is eventually brought out into the open. The anguish of Krogstad's blackmail starts the process, but the final blow is Torvald's reaction when he finds out the truth. When what Nora deems "the wonderful thing" doesn't happen—when Torvald fails to attempt to sacrifice himself for her—Nora realizes that their relationship has been empty. The love she imagined never existed.
There was never any chance of her experiencing the wonderful thing she'd hoped and feared for. She tells her husband, "Our home has been nothing but a playroom. I have been your doll-wife, just as at home I was papa's doll-child" (3.286). In the end, Nora has a sort of spiritual awakening. She walks out into the night alone but, for perhaps the first time in her life, she's on the path to becoming a fully realized, fully independent human being.
Go get 'em, Nora.