Alex Krycek - conspirator and Russian agent

FBI Special Agent Alex Krycek was originally assigned to Mulder as his new partner. He worked with Mulder to investigate Scully's disappearance, until it became increasingly obvious that Krycek was in fact working with the conspiracy.

Kycek's involvement with aliens has become increasingly complex. Surviving an assassination attempt, he went freelance, became infected with Black Oil, was revealed as a Russian agent, lost an arm, trafficked in Black Oil victims, blackmailed Skinner, and killed off his old boss. Oh, and he shot Mulder's father and Scully's sister. He's not very nice, really.