Nora and Christine move in directly opposite paths over the course of the play. Christine begins as a woman who is alone without any obligations to family; Nora has a dutiful husband and several children. While Christine has lived a life of toil and hardship, Nora has had it relatively easy. Nora did have to work a few odd jobs and skim from her allowance to make payments on her debt over the years, but that hardly compares to the sheer drudgery that Christine's life has been.
When the play concludes, the two women have switched places. Nora deserts her family and walks out the door into a life of hardship, whereas Christine unites with Krogstad to form a new, hopefully happy home. By the end of the play, Nora is on the road to becoming the wise woman of the world that Christine already is.
For the most part, Torvald and Krogstad stand out in stark relief to each other. Torvald has a bright future and the respect of the community; Krogstad's prospects are pretty dismal and everybody hates him. Torvald is a law abiding do-gooder while Krogstad is a shady blackmailer.
When Torvald is willing to submit to Krogstad's blackmail, we see that all his talk of being honest and forthright was really just talk. Torvald is ultimately just concerned with appearances. In this way, he's actually similar to Krogstad, who'll do anything to regain his standing with the community. Still, despite Torvald's numerous flaws, you don't see him resort to blackmail. Life hasn't forced him to sink to the levels that Krogstad has.
It's interesting that Krogstad is Torvald's childhood friend, just as Nora's foil, Christine, is a friend from earlier days. Ibsen sets up a direct parallel between the two pairs. Krogstad is like the dark version of Torvald, the version that's seen real hardship. Nora and Christine could be compared similarly. Also, just like Nora and Christine, Torvald and Krogstad move in opposite paths. Torvald ends up alone, whereas Krogstad is about to embark on a hopeful future.