John might be the original Stephen, a lawyer who is interested in robotics. This is Francis Quinn's theory: that after his car accident, John builds a robotic replacement for himself. And what we see of John seems to fit in that theory. He's crippled and works closely with Stephen.
For instance, John goes off to a country place for two months (the amount of time Lanning said it would take to build a human-seeming robot (159)); and when he comes back, he tells Stephen that his part of the plan went well (242-3). That makes it seem as if John has built a human-seeming robot for Stephen to hit in public.
Well, we can't know for sure if John built Stephen, but that's one possibility. And everything we learn about John Byerley can fit in with this theory.