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).
Robert Havard
In addition to his medical research papers, Havard authored an appendix for C. S. Lewis's "The Problem of Pain" as well as a description of Lewis included in "Remembering C. S. Lewis: Recollections of Those Who Knew Him" and one of J. R. R. Tolkien included in "Mythlore."