What is the relationship between Screwtape and Wormwood in The Screwtape Letters?

The relationship between Screwtape and Wormwood is primarily that of a teacher and a pupil. Screwtape is the knowledgable superior to whom Wormwood must listen as the older demon instructs him in the devilish art of leading men from the path of righteousness.
At the same time, the relationship between the two would-be corrupters of humankind is, on the face of it, a good deal more affectionate than the average teacher-pupil relationship, being as how Screwtape is Wormwood's uncle. For the most part, Screwtape's letters are written in an appropriately avuncular tone as he insistently, but patiently, enjoins his incompetent nephew to work harder and smarter in leading his patient towards Satan.
But before long, Screwtape's patience with Wormwood begins to wear thin, and a more querulous note creeps into their correspondence. For his part, Wormwood resents being told what to do by his domineering uncle and tries to turn the tables on him by ratting him out to the Secret Police, which only serves to make Screwtape even more angry.
In the final letter, Screwtape claims that, despite everything, he still loves his nephew. But it's clear from his subsequent comments that this is a very funny kind of love indeed. For this is not the affectionate love between two relatives, nor the kind of Christian love that the patient displays and which Wormwood tries, but fails, to suppress, but rather the love of a glutton for a dainty morsel.