Mulder's obsession began as a result of the alien abduction of his sister Samantha when Mulder was 12. He pursues his cases with reckless abandon for the law, often finding himself suspended or under investigation for disorderly conduct in his quest for "The Truth."
The FBI, suspicious of his activities, paired him with former doctor and current agent Dana Scully. A hard-bitten skeptic, Scully slowly becomes convinced of the truth of the Conspiracy after spending years with Mulder. The two later become romantically involved in the last season of the series, though Mulder clearly carries feelings for Scully for years before they express attraction.