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One approach that you may wish to take in an explication of the relationship of Estella and Pip of Great Expectations may be to explain how this relationship illustrates the predominate theme of Appearances vs Reality in the narrative of Dickens. For, Pip certainly does not follow the advice of Mr. Jaggers who exhorts him to "Take nothing on appearances." Instead, he assumes that Estella is a young lady and he is her inferior. He perceives Estella as something stellar, beautiful, and perfect to which he must attain. He also assumes that Miss Havisham intends for him to be betrothed to him despite her cruelty to him because as a young lady, Pip is the one always called upon to pick up Estella at the station. Despite all his kindnesses to Estella, she is not selected for Pip, nor does she care for him; she does not even care for herself. However, when Pip sees Estella many years later, he remarks,
The freshness of her beauty was, indeed, gone, but its indescribable majesty and its indescribable charm remained....Those attractions in it, I had seen before; what I had never seen before was the saddened, softened light of the once proud eyes; what I had never felt before was the saddened, softened light of the once proud eyes; what I had never felt before was the friendly touch of the once insensible hand
She tells him she is greatly changed, speaking to him in a voice of "touching interest to a wanderer." She tells Pip that suffering has taught her, and Pip feels that her suffering has given her the ability to understand what his "heart used to be." Estella and Pip become friends, not lovers.