For anyone to say that a character in a novel is not important is ignorance. When analyzing a film/novel/any work of art, we are trained to pay attention to even the slightest detail (we’re studying the length of shots for Christ’s sake!). How can we rule out a character? We shouldn’t even be wasting our time asking which character is more important. And as far as saying a character plays off another, that’s blatantly obvious. Practically all characters in all novels are different in some way. That’s how we learn from the characters actions and thoughts, by comparing them to others.
Little Women does a great job of writing about five women in different stages of their lives. By having such a large number and giving them distinct personalities, a larger audience can find someone to relate to more easily. Also, we see different ethics to strive for: freedom of expression, etiquette, caring for others over yourself, etc. All of these characteristics are split up between the four daughters, but all appear to be within Marmee. She is the ultimate woman in some senses, as Mom’s are generally seen in the eyes of their offspring. Marmee nurtures and cares for each girl in a separate, different way which shows her range of character. When can we start talking about her?