Fox Mulder believes. He believes in aliens, in monsters, in the paranormal, in the supernatural. He believes that the unexplained has an explanation that defies logic. He believes in a vast, complex government conspiracy designed to cover up the existence of aliens and their abduction of humans. It is that conspiracy that led to the disappearance of his sister, Samantha. That belief, that desire to find out what happened to his sister, is what has driven him and his life's work. It has brought him into conflict with his family, friends, work colleagues and with his partner and closest confidant, Dana Scully.
However, Mulder is no wide-eyed conspiracy lunatic. He has a degree in psychology from Oxford University in the UK. He was a rising star in the FBI's Behavioral Science Unit, where he impressed his superiors with his ability to psychologically profile criminals. When he discovered the X-Files, his life changed. He found a way he could find out what happened to his sister and expose the people responsible for her disappearance.
Sadly, things don't always work out that way. Mulder was finally able to figure out what happened to his sister, though no one was ever brought to justice for her abduction (naturally, the Cigarette Smoking Man was behind it). He was also never able to expose the government conspiracy covering up the existence of aliens. The final episode finds him on the run from the law, though the 2008 movie sees him absolved of any crime and brought back into the fold of the FBI