When Pip describes Biddy, it is always in glowing terms. When he describes Estella, it is painful.
Voice is how an author uses words. Dickens uses voice to show how Biddy and Estella are different.
Biddy and Estella come from different backgrounds, even though they were of basically the same social class before Estella was adopted. Pip is not interested in Estella just because of her class. To him, she is the epitome of a beautiful lady. Biddy and Estella are so different in personality that they are complete opposites, and Pip does not realize until later in life that he would rather have someone like Biddy than someone like Estella.
While Estella is cold and mean, Biddy is warm and compassionate. Pip can confide in Biddy, whereas he can barely have a conversation with Estella.
From a young age, Biddy is Pip’s friend and confidante. She is his first teacher, and infinitely patient with him.
I reposed complete confidence in no one but Biddy: but I told poor Biddy everything. Why it came natural for me to do so, and why Biddy had a deep concern in everything I told her, I did not know then, though I think I know now. (ch 7, p. 67)
Pip never describes Biddy in any kind of romantic way. He is critical of her, even before he is old enough to know about social class. He also directly compares her to Estella.
She was not beautiful—she was common, and could not be like Estella—but she was pleasant and wholesome and sweet-tempered. (ch 17, p. 88)
Pip says that “in my first uneasiness and discontent I had turned to her for help, as a matter of course” (p. 89).
Biddy is often described in near-saintly terms. When Pip tells her that he can never be happy or comfortable “unless I can lead a very different sort of life from the life I lead now” she tells him it’s a pity. She is trying to tell him to be happy with who he is. Biddy is the voice of reason, but Pip does not listen.
To Dickens, Biddy is the perfect girl. She is kind, compassionate, and patient. She is happy with her status and her position in society.
Estella is actually Magwitch’s daughter. She is adopted by Miss Havisham because she wants a girl to use to get revenge on men. She raises her to be a perfect snob, and Pip is completely baffled by her.
“With this boy? Why, he is a common labouring-boy!” (ch 8, p. 42)
Pip says that her contempt “became infectious” and I caught it” (p. 42). He begins to have as much contempt for himself as she has for her. Dickens uses Estella to demonstrate how the upper class feels about the lower class.