The fictitious MacPhee varies greatly in three novels attributed to C. S.Lewis. In the first, Perelandra (1943), his name is spelled McPhee and he is mentioned only once: “… a sceptical friend of ours called McPhee was arguing against the Christian doctrine of the resurrection of the human body. I was his victim at the moment, and he was pressing on me in his Scots way…” This suggests that McPhee is a Scot, but Lewis probably means merely that McPhee argued like a Scot.
In That Hideous Strength (1945) Lewis emphasizes that MacPhee is an Ulsterman with the Ulster-Scots dialect. He is a major character whose name is spelled MacPhee.
In The Dark Tower (1938 or 1958) his name is again MacPhee. But on the first page he is a Scot: “Go on, go on,” said the Scot…” According to Roger Lancelyn Green, MacPhee had appeared as “a far more interesting and lifelike character in That Hideous Strength.”
C. S. Lewis’s tutor William Kirkpatrick was an Ulster Scot, and Lewis scholars all seem to agree that Lewis based MacPhee on Kirkpatrick. Most Americans mistakenly assume that an Ulster Scot is a Scot; but Ulster Scots are certain Northern Irish who speak the unique Ulster-Scots dialect. (They are also noted for being Presbyterian.)
According to Ulsterman James O’Fee, to their neighbors (including Belfastmen like Lewis) Ulster-Scots speech is distinctive and faintly comic. “Of couse, Kirkpatrick would not have spoken pure dialect, but his speech would have had that colouration. Lewis uses that dialect in That Hideous Strength by giving MacPhee amusing expletives such as ‘forbye’ (meaning ‘as well’).”
If The Dark Tower was written by an American rather than a native of Ulster, that would explain the writer’s assumption that MacPhee was a real Scot rather than an Ulster Scot. And of course it would explain the change in his character noted by Roger Lancelyn Green.