Magwitch is not introduced by name in Chapter One of Great Expectations but is obviously an escaped convict wearing a prison uniform. From Pip's point of view the convict is a terribly ferocious monster, but the reader can see that the man is really frightened, desperate, hungry, and cold. Later, in Chapter 39 when Magwitch reappears and reveals himself as Pip's benefactor, he refers to himself as "that there hunted dunghill dog wot you kep life in." The convict's speech shows him to be ignorant and at the bottom of the social ladder.
“You bring me, to-morrow morning early, that file and them wittles. You bring the lot to me, at that old Battery over yonder. You do it, and you never dare to say a word or dare to make a sign concerning your having seen such a person as me, or any person sumever, and you shall be let to live. You fail, or you go from my words in any partickler, no matter how small it is, and your heart and your liver shall be tore out, roasted, and ate."
Of course, the reader knows the convict is bluffing. He is in no position to do anything to Pip. The reader even feels some compassion for this fugitive who seems to be all alone against a hostile and pitiless world. In fact, the reader is glad to see that Pip brings the poor man what must seem like a banquet the next morning and can imagine how good that brandy must taste--and feel--to a man wearing nothing but a thin prison uniform in that awful cold. Pip's description of the convict's eating further characterizes him as a member of the very dregs of society.
I had often watched a large dog of ours eating his food; and I now noticed a decided similarity between the dog's way of eating, and the man's. The man took strong sharp sudden bites, just like the dog. He swallowed, or rather snapped up, every mouthful, too soon and too fast; and he looked sideways here and there while he ate, as if he thought there was danger in every direction of somebody's coming to take the pie away. (Chapter 3)
Altogether the impression created by the convict makes it unthinkable that he could ever be in a position to reward Pip as sumptuously as he actually does some years later. Magwitch, as the convict's name turns out to be, thought Pip was being "noble" in bringing him so much to eat and drink. He was so frightened himself that he didn't realize how frightening and convincing he must have seemed to the little boy when he threatened to tear out his heart and liver. Magwitch turns out to have many good qualities which were not apparent in the first chapter. Although he is totally ignorant, he has the intelligence to make a fortune in Australia starting with nothing but his bare hands.