Who is Mme. de Guermantes from Remembrance of Things Past and what is their importance?

Mme. de Guermantes is Robert de Saint-Loup's aunt. The narrator is fascinated with her and goes out of his way to run into her in public. He is in love with her and she hates him. Once he no longer loves her, she grows intrigued with the narrator because of his friendship with Robert and Mme. de Villeparisis, and she becomes his friend. The narrator criticizes her poor taste in art.