Screwtape is the central narrator of C.S. Lewis's The Screwtape Letters.
The 1942 book is presented as a series of letters from Screwtape, a senior demon, to his inexperienced nephew Wormwood. Screwtape advises Wormwood on tempting a human patient and distracting him from Christianity. Lewis uses this imagined correspondence to examine moral weakness, spiritual temptation, pride, and self-deception.
Wormwood is an important character, but he is the recipient rather than the letter writer. That distinction makes Screwtape the canonical answer. Aslan belongs to the Narnia stories, while Ransom is the surname of the protagonist in Lewis's Space Trilogy.
Lewis wrote the work during the Second World War. The letters first appeared in the Anglican newspaper The Guardian, which should not be confused with the modern newspaper of the same name in discussions of the book's publication history.