How is the relationship between Pip and Magwitch presented in Great Expectations by Charles Dickens?

When Magwitch returns, Pip is first ashamed of him, but then grows to care about him. 

Dickens presents the relationship between Magwitch and Pip as complicated, starting from when Pip was a small boy and Magwitch was a convict. As Pip becomes a gentleman, he grows to resent everything about where he came from. He is too good for his former life. When Magwitch returns, it reminds Pip of where he came from.

Magwitch and Pip’s relationship began with a chance encounter. Pip ran into Magwitch at a cemetery when Magwitch was on the run after having escaped from prison. Magwitch threatened Pip, but he was not particularly violent. If anything, Pip felt sorry for him. 

At the same time, he hugged his shuddering body in both his arms—clasping himself, as if to hold himself together—and limped towards the low church wall. As I saw him go, picking his way among the nettles, and among the brambles that bound the green mounds, he looked in my young eyes as if he were eluding the hands of the dead people, stretching up cautiously out of their graves, to get a twist upon his ankle and pull him in. (Ch. 1)

Magwitch remembered the kindness Pip showed, and when he made his fortune after being deported to Australia he sent money to Pip so he could become a gentleman. 

As brief as his stay with the convict is, it has a big impact on Pip. Whenever he encounters a convict, he wonders if the man knows Magwitch. When Magwitch shows up at his door in London one day, Pip is repulsed by him. 

He ate in a ravenous way that was very disagreeable, and all his actions were uncouth, noisy, and greedy. Some of his teeth had failed him since I saw him eat on the marshes, and as he turned his food in his mouth, and turned his head sideways to bring his strongest fangs to bear upon it, he looked terribly like a hungry old dog. (Ch. 40) 

Magwitch is entranced with Pip and sees him like a son. Pip is horrified not just because of Magwitch’s manners, but also because Magwitch is a criminal. As a gentleman, Pip can’t imagine associating with Magwitch. Magwitch is also in constant danger because he was not allowed to return to England. Pip becomes responsible for hiding Magwitch and then trying to get him out of the country. 

Magwitch’s past catches up with him. He is injured while trying to escape his old partner Compeyson's attempt to turn him in to authorities. While Magwitch is in jail dying, Pip softens toward him. He realizes Magwitch really does care about him, and he begins to care about Magwitch. Pip makes sure Magwitch dies comfortably wth the knowledge that his daughter is alive and well. Pip does not have the heart to tell him how Estella really turned out.