Bio: Mr. X or just plain 'X' is a mysterious character followed in the footsteps of Deep Throat in providing Fox Mulder with information on covert operations (in seasons two through four). This shadowy and dangerous man alternates as an informant and an alerter to Mulder possibly doing this in return for the debt he owed Deep Throat.
Mulder initially contacted X by making a masking tape 'X' in his apartment window and shining a desk lamp through the window. Unlike Deep Throat, X's motives are hard to establish (and are never mentioned) - his edginess and actions suggests that he has his own agendas of which Mulder is only a part. Sometimes it seems as if he is stringing Mulder along in a complex and devious campaign of disinformation. At other times he acts as an ally in the search for the truth.
He went head-to-head with Assistant Director Skinner after he refused Agent Scully's request for information on Mulder's whereabouts in End Game. He also saves Mulder from a burning train in 731. In Soft Light he took an even more active yet contrary role, stepping out of the shadows to intervene directly in Scully and Mulder's case, spiriting away a key figure under Mulder's very nose. Angrily, Mulder told X he wanted nothing more to do with the man. In Wetwired X played an even more dangerous game using Mulder and other agents as pawns believing that like chess pawns, they were expendable. Mulder was allowed to live, still with no clue as to whose side X was on. It was revealed, however, that X worked for The Cigarette-Smoking Man, but may have his own agenda. On more than one occasion he has refused to risk his life for Mulder, stating that he did not want to meet the same fate as his predecessor, Deep Throat. However like all Mulder-helpers he too meets the same end.
In Talitha Cumi, he takes surveillance photos of a meeting between The Cigarette-Smoking Man and Mulder's mother. The Cigarette-Smoking Man however finds out about these pictures and lays a trap for what he has learned is a leak in their organization, the Consortium.. In Herrenvolk this trap is sprung and X is tricked into revealing his betrayal, leading to him being fatally shot in the stomach on Mulder's doorstep. With his final breath X writes 'SRSG' on Mulder's doorstep, leading him towards a new player…one more informant in the string of Mulder-helpers, Marita Covarrubias. X's role has never truly been explained.

• "The truth is out there"

• "You're choosing a dangerous time to go it alone."