John Kresge (played by John Pyper-Ferguson) was a detective in San Diego who worked on the murder investigation of Marshall Sim's wife. During the Scully family Christmas reunion, Dana Scully became involved after receiving unusual phone calls that sounded like her deceased sister Melissa. Scully found herself becoming more and more involved with the case, particularly in regard to Emily Sim. Kresge assisted Scully during the investigation despite his initial annoyance at Scully's constant presence near what Kresge deemed a local matter not suited for the FBI (TXF: "Christmas Carol").
However, Kresge became more helpful upon the discovery that Emily was Scully's biological daughter and part of a larger conspiracy. Mulder and Kresge were seeking Doctor Ernest Calderon when they were approached by a member of the hybrid police disguised as Calderon. Mulder warned Kresge that his gun would be useless, but Kresge shot the hybrid anyway and was rendred ill by his toxic blood. Later dialogue establishes that Kresge survives (TXF: "Emily").