Dickens uses Estella Havisham, the adopted daughter of the rich Miss Havisham, to demonstrate Pip's constant wantings for a richer, better life. It is through her anatoginizing of Pip and her letting him know where his place is in society that the audience gets a glimpse into how much Pip wants to change his lot in life and become a gentleman and all that comes with that title.
Mr. Pumblechook, Joe Gargery's Uncle, proves to be a catalyst for Pip wanting to become a bonafide gentleman and not to settle as a simple imposter as Mr. Pumblechook.
Biddy, Pip's early teacher, was simply a female version of Pip--an orphan with no high station in life. However, Dickens uses Biddy as a reflection of Pip and what he alone disdains about his life. Despite Biddy's obvious love for Pip, Pip has no urge to marry her because she does not reflect his want to raise his station in life. Once he realizes that Biddy would suffice as wife, it was too late. Biddy had moved on. It is Pip's stubbornness and his dreams of being a gentleman that drives his decision to remain blind to Biddy's initial love and in the end, leaves him loveless.
Magwitch, the escaped convict that Pip initially meets as a scared, seven year old boy, is used at the end of the novel to demonstrate Pip's ability to be compassionate. It is Magwitch's death sentence and thus, his automatic loss of the great inheritance that would rob Pip of his "great expectations" of being a gentleman, that brings forth the reader's realization that Pip has changed from a man obssessed with being a gentleman to a man satisified with his place in society. Instead of cursing or leaving Magwitch to die alone after his loss of the inheritance, Pip stays with magwitch and holds his hand until Magwitch dies.