FOX MULDER, the lead investigator in the X Files division, is something of a renegade. With an honors degree in criminal psychology from Oxford, the FBI Academy recruited him and he graduated top of his class. He then joined the FBI as a criminal profiler in the Violent Crimes Unit where he aided in the capture of several murderers and develops a promising reputation in the Bureau.
However that all changes because of an event in his life. The disappearance of his younger sister, Samantha under mysterious circumstances when he was 12 was the defining point in his life. However he seemed to have moved on. When on the trail of a government fugitive, Suzanne Modeski, (Unusual Suspects) he is exposed to a large amount of paranoia inducing chemical in a shootout. The government was testing the potency of this chemical that the Russians had developed in small quantities however Mulder gets exposed to far too large an amount. His behavior becomes increasingly erratic and under the effect of this chemical while undergoing a hypnotherapy session by Dr. Heitz Werber, he starts believing that aliens abducted his sister.
It is this event that causes him to grab at the opportunity to work in the X Files division where unexplained cases are handled. After this his reputation goes to the dogs. Called Spooky by his colleagues, he is often thought to be a prime case for dismissal and the continuance of the X Files division is threatened. However Mulder seems to have some mentors in high places, sympathetic to his cause and antagonistic to his opposition. One of these mentors is later found to be Senator Matheson.
However to keep him in control and debunk his theories, his superiors find a way in the form of Agent Dana Scully, a forensics instructor from the Quantico Academy of the FBI. With a strong scientific background and an impressive record, they hope she can do their dirty work. Their hopes are belied as over the time they have worked together, Dr. Scully is the only one Mulder really trusts and respects. And worse, for the Consortium and the bureau hierarchy, the feelings are mutual. Far from debunking his ideas, Scully gives his theories a scientific credibility and helps solve cases--albeit differently -- providing hard evidence to back up his ideas where he thought no proof was needed. Together down in "the basement office with no heat", she helps him solve the supernatural and paranormal cases they come across.
Mulder's faith is in the existence of the truth and the existence of his sister. It is in search of both these that he continues to pursue the elusive despite grave risk to his and Scully's lives. Never willingly putting her in danger, it is very much part of the job. Scully willingly follows him despite the loss of her sister and daughter and attacks on her life, believing his cause to be just.
Fox Mulder is the son of William and Tina Mulder. His father worked in the State Department and apparently on a top-secret project with some of his colleagues who later become a common part of Mulder's life -- either as enemies or as informants. Mulder grew up in a family where there was much strain and insecurity, with his mother always blaming his father for the loss of Samantha. They later separate. Cold and stern Mulder's relationship is always distant. This aspect is never truly explained till Mulder's mother explains to him that Samantha was taken as a guarantee for William Mulder's silence on the project he was working on. When William Mulder again threatens to reveal to his son his role and the contents of the project, the people he had worked with and who wanted to ensure the security of the covert operation -- the Consortium or Syndicate, shoot him dead.
One of the main members of the Consortium is the Cigarette Smoking Man - in charge of Consortium security. His relationship with Mulder is strange. He was a colleague of William Mulder and it has been hinted that he had an affair with Tina Mulder leading to rumors that he is actually Fox's father. He is also supposed to be the man who asked William Mulder to choose between his two children - a choice he passes on to his wife who refused to make a choice and always hates him for it later. While CSM generally tries to get in Mulder's way - it is on his orders that Scully is assigned to the X Files and later it is he who orders her abduction – he also protects Mulder’s life by convincing the Consortium that Mulder as a martyr would be more dangerous. Mulder however feels only one emotion for him – hatred. Mulder is now convinced of the government’s involvement in a covert operation. He believes their policy is to deny everything and if they are caught, apologize.
Mulder has often been double-crossed and cheated by his opposition within the Bureau. One such time is when he is assigned Alex Krycek as a partner (Sleepless). Krycek pretends to be the perfect protégé and partner but soon shows his true colors -- we find that he is working for the CSM to break up Mulder and Scully and when he finds that he cannot succeed in doing so, he recommends more radical action which leads to the abduction of Scully…an act in which he plays no small part. He almost kills Mulder in the process. However Mulder does have his allies – the Senator, the Lone Gunmen, Deep Throat, X, Marita Covarrubias, Agent Pendrell. While some have wavering loyalties and some have been killed because of their association with him, he moves on with or without them.
On a personal level, Mulder is a non-conformist as well - whether its eating sunflower seeds (a habit he picked up from his father), wearing the tackiest of ties, indulging in his interest in aliens or girlie mags or adult videos or never sleeping on a bed (only his couch) unless its in a motel and never asking for directions even when he’s lost….well that’s conformist at least for the male species. He hates being called Fox by anyone, even his parents – only his mother and Mrs. Scully call him by his first name. Even his love interests refer to him as Mulder – and they have been a few of them – Phoebe Green (Fire), Kristen (3), Bambi Berenbaum (War of the Corpopahges), Diana Fowley (who started him off in the X Files division and who returns to take it over again).
Even Scully, who is the closest to him, calls him Mulder. In return, he refers to her as Dana very rarely. Willing to give up his life for her, he often feels guilty for what he has asked her to give up. He has given up quite a bit himself, almost dying only to be revived by Navajo Indians (Blessing Way). He even finds his sister, gives her up to save Scully’s life and then discovers that the person he saw was actually a clone. Mulder now believes that the Consortium, a covert group knows about the existence of extraterrestrials and is helping them develop alien-human hybrids in the hope that when there is an attempt by the aliens to colonize the earth, these hybrids will survive. They use the DNA of abducted people to create these clones. When they discover that the aliens are closing in on them, they join hands with them on the surface while in reality they are only trying to find their weakness.
Mulder comes across a Defense Department employee Michel Kritschgau who for a while convinces him that this belief in aliens and colonization is all a hoax and he was being used to perpetuate it. However Mulder soon regains his beliefs. His mottoes in life include: The Truth is Out There, Trust No One (trustno1 is also his computer password) and I Want To Believe.

• Scully: "You're in the basement because they're afraid of you, of your relentlessness and because they know that they could drop you in the middle of the desert, and tell you the truth is out there, and you'd ask them for a shovel!"
  Mulder: "Is that what you think of me?"
  Scully: "Well, maybe not a shovel. Maybe a backhoe." (Piper Maru)

• "I even made my parents call me Mulder" (Tooms)

• Mulder: "If there's an ice tea in that bag, could be love,"
  Scully: "Must be fate, Mulder. Root beer." (Tooms)

• Skinner: "What do you possibly hope to find Agent Mulder?"
  Mulder: "Why they killed my father, what happened to my sister, and what they did to Agent Scully." (Paper Clip)