Christine Lind is a foil to Nora. Because Nora is the main character, and it is her development that the play is about, Christine is brought in to demonstrate the obvious differences. It is when Kristina comes to see her old school friend that it really becomes clear to the audience what is meant by the play's title; that Nora is living in a “doll’s house," a specially created, protected environment far from the real world. This is emphasized by what we learn of Christine's life; hard-working, adult and responsible. By contrast Nora is not even able to look after her own children, let alone earn her living. Her journey from independence to marriage is a foil to Nora's journey in the opposite direction.
“Mrs. Linde. How kind you are, Nora, to be so anxious to help me! It is doubly kind in you, for you know so little of the burdens and troubles of life.
Nora. I--? I know so little of them?
Mrs. Linde [smiling]. My dear! Small household cares and that sort of thing!--You are a child, Nora.
Nora [tosses her head and crosses the stage]. You ought not to be so superior. "
The characters of Krogstad and Torvald are also in the roles of FOIL's for each other. One is married and the marriage is ending while Torvald is not married yet. The links below can supply specifics. Don't have enough room here.