Biddy's greatest qualities are her intelligence and her empathy. She is someone who always cares for others, while deprecating her own troubles. In many ways, Biddy would have made a more deserving hero for the book than Pip, who tends toward self-pity, at least after he begins to visit at Miss Haversham's. In Chapter 17, Pip tells Biddy about his plan to become a gentleman and of his love for Estella:
“We talked a good deal as we walked, and all that Biddy said seemed right. Biddy was never insulting, or capricious, or Biddy to-day and somebody else to-morrow; she would have derived only pain, and no pleasure, from giving me pain; she would far rather have wounded her own breast than mine. How could it be, then, that I did not like her much the better of the two?”
Biddy, of course, knows Pip's heart better than he does himself (and everything else: earlier in the chapter Pip remarks, "In short, whatever I knew, Biddy knew") and is characteristically understanding:
“Biddy," said I, when we were walking homeward, "I wish you could put me right."
"I wish I could!" said Biddy.
"If I could only get myself to fall in love with you,—you don't mind my speaking so openly to such an old acquaintance?"
"Oh dear, not at all!" said Biddy. "Don't mind me."
"If I could only get myself to do it, that would be the thing for me."
"But you never will, you see," said Biddy.
Biddy, who is smarter than Pip both intellectually and emotionally, is prevented by her insight from hoping that Pip might love her. She is a rare mixture of emotional acuity and rational clear-sightedness. The qualities that make her ill-suited (or too good?) for Pip make her an excellent nurse and confidant.