William was the son of Agent Dana Scully, born in the episode "Existence," out of Democrat Hot Springs, in Northern Georgia. Throughout the last two seasons of The X-Files William was encompassed by an aura of mystique, ranging from his origin to the supernatural abilities he possessed.

Toward the end of the eighth season, the informant Knowle Rohrer states that an implant related to Scully's mysterious abduction was likewise related to her pregnancy. It was also insinuated that Agent Fox Mulder was the biological father. At some point, Scully made the decision to have a child on her own through in-vitro fertilization, and asked her FBI partner, Fox Mulder, to be the donor, to which he agrees. This was revealed via flashback in the episode "Per Manum." It is strongly implied in this same episode that the in-vitro attempt failed and it is unclear exactly when in the X-Files timeline this took place. It was also implied in several instances that Scully and Mulder began a relationship late into their partnership. In "Essence", Mulder's inner monologue reflects: "How did this child come to be? What set its heart beating? Is it the product of a union or the work of a divine hand?" suggesting William may have been conceived naturally, supposedly impossibility for Scully, or by some paranormal intervention, or perhaps both. By the show's last episode, both agents refer to William as their son. However, it is not made clear whether or not this is a term of affection on Mulder's part or a biological fact. Incidentally, William had noticeably wide-set nostrils; perhaps intended to resemble Mulder in this regard.

The New Syndicate frequently has William and Scully under surveillance. During Season 9, William turns out to have some paranormal abilities, including telekinesis. After a visit from a disfigured Jeffrey Spender, William is injected with a magnetite substance, and, according to Spender, seemingly cured of his paranormal abilities. Because, despite all her efforts, Jeffrey Spender was able to gain access to Baby William and inject him, Scully decides that the only way to keep her son safe is to give him up for adoption, to a Christian couple in Wyoming. This occurs in the episode "William." William appears to be around 10-12 months of age at this time.

Scully indicates that he was named after Mulder's father, who was named William, although, coincidentally Scully's father was named William as well.

The origins of his powers are never explained. It is possible that he is similar to Gibson Praise, and has otherworldly powers. He displays telekinesis, by causing a mobile over his crib to revolve.

Chris Carter has stated that the topic of William will be covered to some extent in the 2008 film The X-Files: I Want to Believe.

Credited to Wikipedia, the free encyclopedia http://en.wikipedia.org/wiki/Category:The_X-Files_characters