How does Xury's character provide context in Robinson Crusoe?

Xury's character in Robinson Crusoe provides context within the novel by showing him as subordinate to Crusoe, a white European.

To a considerable extent, the asymmetrical relationship between the two represents in microcosm the power imbalance that existed between white Europeans and indigenous people at the time when the book was written.

It's made clear from the very first encounter between Crusoe and Xury that the young African boy is in a subordinate position to Crusoe. Although some kind of friendship arises between the two, it's not an equal friendship by any stretch of the imagination. Such is the enormous cultural gap between them that Crusoe can never regard Xury as his equal, and equality is surely the foundation of any true friendship.

Despite the fact that Xury helped Crusoe attain his own liberty, he eventually sells the young boy into indentured servitude for ten years. Though this isn't strictly speaking slavery, it's not far off; and to add insult to injury, Xury will be set free if he converts to Christianity, reinforcing once more the dominance and control that the white man has over indigenous people.

Once again, we see Xury as the representative of colonial subject peoples and the appalling treatment they receive at the hands of their self-proclaimed masters.

How does Xury's character and fate advance the plot in Robinson Crusoe?

In Daniel Defoe's Robinson Crusoe, Xury's character and fate advance the plot of the story by showing readers something important about power within the story and about Crusoe's character.

Xury is a young boy Crusoe meets when the latter is taken captive by a Turkish pirate. They are both slaves to the pirate, but Crusoe keeps Xury with him when he escapes from slavery. He seems to find the boy trustworthy, yet Crusoe also believes himself to be superior to this boy, who is not white and not Christian.

When the Portuguese captain picks up the two fugitives, the captain automatically assumes that Xury is Crusoe's slave, and he offers to buy the boy from Crusoe. Crusoe does not really want to sell Xury and deprive him of the liberty he has just gained, but he is willing to let the boy stay with the captain. Notice that Crusoe believes he has the right to make this decision. So Crusoe makes a deal (without consulting Xury, of course). He will indenture the boy to the captain for ten years, and then, if Xury converts to Christianity, he will be free.

We can see, then, that Crusoe and the captain are the powerful ones in the story. They determine Xury's fate for him. Despite Xury's loyalty to Crusoe, the latter does not treat the boy as an equal or as a person who can and should determine his own fate. We might think that Crusoe should have learned something during his own time as a slave, but apparently, his ideas are too deeply engrained, and Xury is stuck with the decisions others make for him.