X (The X-Files)

X first appears at the beginning of season 2, after he contacts Mulder to tell him he "has a friend at the FBI." X became Mulder's information source after Deep Throat, Mulder's first informant, was executed. Cigarette Smoking Man was X's superior.

X was quite different from his predecessor. He was much more cautious, far less friendly, and being an MIB operative, rather ruthless. Often he would not respond to Mulder's signal for help/information (two strips of tape in the form of an X in Mulder's window), if X felt that the risk to his own life was too high. In his own words, he viewed Mulder as his tool, which he would go to when needed. These facts led to an unstable partnership between Mulder and X, and heated arguments with guns drawn was not uncommon.

While X's loyalties, or what his own agenda was, was often unclear, he has more than once proven that he at least does not want Mulder dead. It is likely that, while he wished certain information to be released for the common good as Deep Throat did, his own perspective was more hardened than Deep Throat's had been. There were at times indications that X cared more about Mulder than he wanted people to believe; he may have felt that it was inappropriate for a member of a group like the Men in Black to be seen as having any emotional attachments, for whatever reason. His voluntary naming of Marita Covarrubias as his successor while dying (and his ghostly appearance to give Mulder her address in the final episode) strongly implied that he felt Mulder's work in exposing the conspiracy was of vital importance; however his precise motivations for this belief remained obscure. His actions during the events leading to his own assassination also implied that, although his outward behaviour was at times ruthless, he was nevertheless possibly capable of a greater level of compassion than would customarily be expected from someone in his position.

In the episode "End Game", he goes to Mulder's apartment but finds Scully, who pleads that she needs to know where Mulder is, believing his life to be in danger. Initially X refuses, and is subsequently confronted by Skinner, who seemed to recognize X. He relinquishes Mulder's location, though not until after a brief but intense scuffle with Skinner, in which X draws his firearm point blank and remarks that he has "killed for far less." (It is odd that he would go to Mulder's apartment and knock on the door even though he knows that Mulder has left the country.) In the episode "731", X's loyalty to Mulder is further confirmed. Trapped on a train car equipped with a time bomb, Mulder, about to escape, is attacked brutally by an MIB assassin, the Red Haired Man. As the Red Haired Man is about to step off, he is shot fatally by fellow Man In Black, X. X boards the car and with only time left to save either Mulder or the alien-human hybrid the car was transporting, he opts to save Mulder, and carries him off to safety just as the car explodes.

X also appears in The X-Files: The Game (which takes place near the end of season 3) and offers the player (Craig Willmore) a weapon used to kill EBE's, and also alludes to the existence of aliens and Scully's location.

In the season 4 opener "Herrenvolk", X's position as an informant is discovered by the Syndicate. When suspicion arises after the finding of photographs that where taken of Cigarette Smoking Man (by X), false information is planted at the Elder's behest, in order to root out the leak. The information, in the Elder's words, was "That Mrs. Mulder's life is in danger... that left unprotected... she may come to an unnatural end." X, attempting to relay the information to Mulder, goes to his apartment, and is surprised by fellow MIB operative, the Gray Haired Man, who guns down X. With his last strength, X crawls to Mulder's doorstep and writes in his own blood "SRSG", meaning "Special Representative to the Secretary General" of the UN, his successor.

After his death, X appears two more times: in the Lone Gunmen origin story "Unusual Suspects" (in which he inadvertently gives the Lone Gunmen their name, by commenting that he heard the JFK assassination was the work of a lone gunman), and as a ghost in the series finale.

Plainclothes Man was his only known subordinate. X eliminated him at the end of the episode "Wetwired". This was carried out by X in order to secure his own safety, as Cigarette Smoking Man was having him watched closely.

Trivia

*X was originally set to be a woman. In fact, the scene where Mulder first meets X, was shot with actress Natalija Nogulich playing the part. However, the production staff felt that it would not work, and so the scene was altered with shots of Steven Williams edited in.
*X's death scene was inspired by Sean Connery's death scene from The Untouchables.
The X-Files: The Game shows a particularly ruthless side of X. The player has one brief meeting with X, and he then begins to leave. If you then chose to follow him, he immediately and brutally kills you, without a word.