X-Connections
Mulder studied psychology at Oxford and joined the FBI as a profiler. In Travelers, which takes place in 1991, he hasn't yet opened The X-Files, so he probably opened them some time at the end of that year, since Scully was assigned to work with him in May of 1992. His main drive for trying to solve the unexplained cases was the hope that he might some day find his sister.
Mulder learns in Paper Clip that it was he who was supposed to be taken. Mulder also learns that his father, Bill Mulder, was a part of the conspiracy, and was even responsible for his sister's abduction. His parents had to make a choice between their children, because each Syndicate member had to sacrifise part of his family as human collateral for the aliens in exchange for valuable information. Mulder's mother, Teena Mulder, tells her son that it was her husband's decision. The Mulders' marriage fell apart and they divorced.
Of course, their marriage couldn't have been doing too well since Mrs. Mulder was having an affair with the infamous Cigeratte-Smoking Man. Samantha is first suggested to be CSM's biological daughter in 4th season's Demons, and is confirmed in the full disclosure(well, kinda, I personally didn't pick it up, but I read in an article before it aired, something to the extent of'we are saying that Samantha is CSM's daughter'). Later, in the second part of the sixth season premiere, The Sixth Extinction II: Amor Fati, it is suggested that CSM is also Mulder's biological father. It isn't really proven until William when it is discovered that Jeffrey Spender, CSM's son, shares DNA with Mulder, and in The Truth, Spender states that he is Mulder's half-brother.
At the end of the 7th season, Mulder was abducted. He was returned and tried to get his life back to normal, which was pretty hard since John Doggett had been assigned to The X-Files while he was away and, of course, Scully being pregnant probably was a shock. Mulder ends up quiting the FBI when he takes the blame for a mishap in Vienen and tells Doggett that "it's his office now."
At the beginning of the 9th season, we find out that Mulder is in hiding because his life is in danger. We also know, after an entire season of waiting, that Mulder is the father of Scully's son, William.
Mulder returns in The Truth, and after being found guilty in a staged hearing, he is helped to escape from prison. He and Scully run off together to live together where she helps hide him from the FBI until early 2008 when the FBI asks for his help (The X-Files: I Want to Believe). Now no longer farced to hide, it's hard to know what Mulder will do next. . .
X-Connections
Characters