Christine is a tough, world-wise woman. If Nora seems like a "little squirrel," Christine seems like a mongoose. If Nora is a "little lark," Christina is a freakin' hawk. This lady has been through a lot.
She tells Krogstad, "I have learned to act prudently. Life, and hard, bitter necessity have taught me that" (3.32). In her younger days, she had to sacrifice love for the sake of her family. Rather than marrying the dashing young Nils Krogstad, she married a businessman, Mr. Linde, so that she could support her sick mother and her two younger brothers. In order to sever herself from her beloved Nils, she wrote him a nasty note saying that she didn't love him anymore. (A little harsh, Christine.)
Now her brothers are all grown up and her mother is dead. Her husband has passed away, too. Mr. Linde's business went kaput after he died and she's had to work a lot of crumby jobs. Still, Christine is finally free.
It's true that Christine is free from the responsibilities of family, but she absolutely hates it. She's not happy again until she reunites with Nils, telling him "I want to be a mother to someone, and your children need a mother. We two need each other" (3.58). Hmm, now that's a pretty interesting thing for a woman to say in a play that's often painted as being a feminist paean.
Here we have a woman who is capable, intelligent, and self-sufficient. Christine is a liberated lady smack dab in the middle of Victorian Europe, and what does she go and do? She willingly jumps back into the role of wife and mother, because it's the only way she knows how to be happy.
What are we to make of Christine's decision to become a part of Krogstad's household? How does this fit into the overall message of the play? It might be seen as tragic: women are so programmed by society that the only thing they know how to do is be a homemaker. On the other hand, it's not like Christine is making this decision from a place of ignorance. Unlike Nora, Christine is well aware of what life is like without men.
The major difference between Christine's new relationship and that of the Helmers seems to be that Christine and Krogstad are entering into it as equals. Christine says to Krogstad,
"Nils, how would it be if we two shipwrecked people could join forces? […] Two on the same piece of wreckage would stand a better chance than each on their own." (3.42-3.44)
Perhaps, the union of Nils and Christine is Ibsen's example of "the most wonderful thing of all," which Nora defines as "a real wedlock" (3.376-3.378). Aww. That's kind of sweet... although we don't suggest you write anything about "we two shipwrecked people" in your next Valentine's card.