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Aw, Joe. We kind of love Joe. He's Pip's brother-in-law and childhood hero, but he's also just a genuinely nice guy. Pip describes him:
a fair man, with curls of flaxen hair on each side of his smooth face, and with eyes of such a very undecided blue that they seemed to have somehow got mixed with their own whites. He was a mild, good-natured, sweet-tempered, easy-going, foolish, dear fellow,—a sort of Hercules in strength, and also in weakness. (2)
That's some nice direct characterization to start us off, and everything that Joe does or says proves Pip's point, like how he told Mrs. Joe to bring Pip to live with them: "I said to her, 'And bring the poor little child. God bless the poor little child,' I said to your sister, 'there's room for him at the forge!'" (7.3), or how he tells the grown-up gentlemanly Pip that "you and me was ever friends" (57.19), or how he pays off all of Pip's debts and then sneaks away in the middle of the night so Pip won't be ashamed of him.
Joe may not be comfortable in the city, and he may act like a dweeb around Miss Havisham, but we don't think he has anything to be embarrassed about. He's basically the perfect man: as Biddy points out, he "ever did his duty in his way of life, with a strong hand, a quiet tongue, and a gentle heart" (35.40). No wonder she marries him.
The thing about Joe—like pretty much all the characters in this book except for Pip—is that he's more of a caricature than a real character. He doesn't have an arc of his own; he's just there as a tool to help Pip grow up by first rejecting him and then starting to admire him again. Sure, he does learn to read and write, but he never becomes what Pip would think of as a gentleman. He doesn't need to. He's already more of a gentleman than most city guys will ever be.