Mulder's greatest nemesis was The Smoking Man, who, despite his obvious ill-intent, seemed to hold Mulder in special regard. Though he mocked Mulder to his face for his foolishness and the futility of his quest, in "Two Fathers", he told his son Jeffrey Spender, "You pale to Fox Mulder." Mulder also developed an intense enmity for Alex Krycek, a double agent who briefly worked with Mulder in the FBI as an infiltrator, and who continued to serve as a recurring adversary, acting as an assassin for the Syndicate in the murders of Bill Mulder and Melissa Scully.