Screwtape is the narrator of C.S. Lewis’s The Screwtape Letters.
The 1942 book is presented as a collection of letters from Screwtape, an experienced demon, to his inexperienced nephew Wormwood. Screwtape offers advice about tempting a human patient, allowing Lewis to discuss moral and spiritual ideas through satire and reversed perspective.
Wormwood is the recipient of the letters, not the narrator. That distinction is the central identification point in this question. The book’s imaginative device makes the reader examine temptation, distraction, pride, and habit from the viewpoint of a fictional enemy.
Lewis dedicated the work to J.R.R. Tolkien, though the two authors had a complicated friendship and later disagreements. The Screwtape Letters became one of Lewis’s most widely read works of Christian apologetics, even though it is structured as literary fiction rather than a conventional theological treatise.