The sorcerer who taught Strange the rules, then found out his own teacher had never followed them, and drew the opposite conclusion.
Mordo is the film’s best structural idea and the one it does least with. He is the true believer: rigid, decent, and certain that the rules exist because breaking them costs something. Then he learns that the Ancient One broke the central rule for centuries, and that Strange broke time itself to win, and he concludes that the problem is not the rule-breaking but the number of sorcerers.
He leaves. His last scene sets up a version of him that has not arrived: the man who decides the world has too many people with power, and starts reducing the number.