Handsome, charming, and well-spoken, the perfidious Agent Alex Krycek (Nick Lea) worms his way into the confidence of Fox Mulder in the second season episode "Sleepless", where he plays his role of worshipful junior FBI agent to the hilt. He even adopts some of Mulder's wry wit ("Well, it puts a whole new spin on 'virtual reality'.") to try to get closer to Mulder.
In Ascension, Mulder trusted him enough to tell him, that he was going to search for Scully.But his true allegiance emerges when he warns The Cigarette Smoking Man, that seperating Mulder and Scully has not stopped their partnershipm and that CSM has underestimated Scully as a danger to their secrets.
In "Paper Clip", the consortium calls Krycek one of CSM's man and in the second season, he is working beneath CSM. His darker side appears more clearly in "Duane Berry" and "Ascension", where he is instrumental in Scully's abduction and the silencing of Duanne Berry. After dissapearing from the bureau, he becomes a greater threat, as he goes underground, and in Anasazi, he murder's William Mulder, Mulder's father, and frames Mulder for the killing. He was one of the two assasins, who were waiting to kill Scully, but killed Melissa Scully.
He ambushed Assistant Director Walter Skinner and beats him up, with a couple of other men, and takes a vital digital tape (which reveals the extent of US government coverup in alien visitations), for which the CSM repays him by arranging his killing and the tape's destruction in a car bomb, which Krycek somehow escaped. In "Piper Maru" and "Apocrypha", he tries to sell the secrets on the tape to the highest bidder. When Mulder finds Krycek again in Hong Kong, Krycek is possessed by the very alien, he has been selling secrets of. The alien uses him to cut a deal with the CSM which leaves him trapped in an abandoned missile silo in North Dacota.
After feeding tips to Mulder on the activities of a militia group in "Tunguska", he joins Mulder in pursuit of a mysterious rock. This leads them to Russia in "Terma" where they are captured and thrown into a gulag. There they are experimented with and some strange substance is injected in them, but Mulder escapes, forcing Krycek to come with him. Krycek escapes only to run into a group of one-armed men that live in the woods, who cut off his left arm, so that the soldies cannot experiment on him.
Next Krycek is seen with a prosthetic, which he learns to use within a few days, in the company of a Russian assasin who had ordered to dispose of those in America involved with the rock. The KGB man had been ordered by one "Comrade Arzten", which turns out to be Krycek.
By "Patient X" and "The Red and the Black" he brings the vaccine to the Well Manicured Man and his friends for a price. By the end of the episode he seems to be firmly on WMM's side. Alex Krycek has been named Ratboy by X-Philes for his faithlessness.
Krycek's freelance status allows him to become the CSM's chief henchman, but his boss turns on him and sends him to a Tunisian prison. CSM eventually has him released for more dirty work, but Krycek gets his revenge. Although Mulder never loses his suspicion of the traitor, he is helped by Krycek in "Requiem" (7X22). The real question is, can Krycek be trusted or is he merely working on his own behalf?