The FBI agent who believes in extraterrestrials in The X-Files is Fox Mulder.
Played by David Duchovny, Fox William Mulder is one of the show’s two central protagonists. He works on the FBI’s X-Files, a collection of unsolved cases involving strange, unexplained, and potentially paranormal events. Mulder’s belief in extraterrestrial life is closely tied to the disappearance of his sister Samantha, an event that drives much of his investigation and personal life.
Mulder is paired with Special Agent Dana Scully, played by Gillian Anderson. Scully is a physician and scientist assigned to apply skeptical, evidence-based reasoning to Mulder’s theories. Their believer-versus-skeptic partnership became the defining relationship of the series, although both characters’ views gradually become more complicated.
John Doggett later takes a larger role in the X-Files office, Walter Skinner is Mulder’s supervisor and ally, and Scully is his original partner—but none is the extraterrestrial believer described in the question. Mulder first appeared in the 1993 pilot and remained the franchise’s best-known investigator of alien conspiracies.