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).
Cecil Harwood
His friendship with Owen Barfield and C.S. Lewis has been well-recorded in the biographies written on these two personalities. It seems to have been a fruitful relationship throughout, influencing the work and thought of all three.