Corporal Walter O’Reilly
Cerebus' Chief of Security. His post is directly outside Cerebus' door. The character is never seen, we only hear him through the door.
From Latter Days notes "The Last Annotations" (p. 252 of Latter Days):
"I had spent a certain amount of time trying to cast the 'chief of security outside the door'. I kept coming back to Radar O'Reilly, the character played by Gary Burghoff in both the M*A*S*H movie and later the television show (the only carry-over actor from the one to the other) both for the fact that he was intrinsically competent and his CO, Colonel Blake was so intrinsically incompetent. I intentionally made him Walter O'Rilly instead of Corporal O'Reilly because it was in the television episode where the field hospital had treated a soldier who believed himself to be Jesus that the character was first given the name Walter, which seemed appropriate to the subject matter.
As I went along, the voice and speech patterns I pictured for the character started changing to that of Smithers, the aide-de-camp (accent on the camp) to Montgomery Burns on The Simpsons. Which also seemed appropriate given the latter's advanced age and malignant personality. Since the character never actually appears "on camera," it didn't seem like a big problem to move rrom one to the other mentally.