At first glance, it is tempting to assume that Christine Linde is essential to the play because she helps the audience "tolerate" Nora's immature and silly behavior. The reason for this assumption rests on the fact that Christine seems to take Nora's annoying behavior quite lightly. In fact, she "mothers" Nora to a point. This helps the audience admit that there may be something about Nora worth liking. We just do not know what it is, at first.
However, on a literary analysis, we realize that Christine is a foil of Nora. Nora is oblivious, happy-go-lucky, immature, bubbly and unreasonable. Christine has gone through hard times, and is quite acquainted with the realities of life in every way. She contrasts Nora quite immensely, which allows the audience to notice the extent to which Nora is actually living in a fantasy world.
Moreover, Christine is actually the only character to whom Nora is honest. She is also the only other female character in the play, which may account for the fact that Nora trusts her as a member of her own gender more so than she would her own husband, because he is a man. Therefore, Christine is the character that allows Nora to come out and tell the truth of what she did for Torval in hopes of getting Christine's own approval.
Conclusively, Christine brings Nora out of her world, and into reality. She listens, so Nora speaks. She comforts, so Nora confesses. She talks about reality, so Nora comes closer to it as well. She is ultimately the only person who shows a real care for Nora, and presents the characteristics that are unique of a real best friend.