Elwin Ransom
Elwin Ransom may be based on C. S. Lewis' friend J. R. R. Tolkien ("Elwin" means "Elf friend" in Anglo-Saxon), though he seems to have autobiographical elements. In "That Hideous Strength" Ransom, with his royal charisma and matter-of-fact breezy acceptance of the supernatural, appears less like Tolkien than like Charles Williams (or some of the heroes in Williams' books).
The Screwtape Letters
The Screwtape Letters is a Christian apologetic novel by C. S. Lewis and dedicated to J. R. R. Tolkien. It is written in a satirical, epistolary style and while it is fictional in format, the plot and characters are used to address Christian theological issues, primarily those to do with temptation and resistance to it.