Bio: Assistant Director Walter Sergei Skinner works for the FBI and is an ex-marine. Stern and efficient, he is given supervisory charge of the X Files. A by-the-book person, he finds Fox Mulder's unorthodox methods of investigation unnecessary. He has little patience with them when they follow leads that are outside their job description and that of the division they work in. Often warning them of the consequences of stepping on the feet of the powerful (Tooms), he finally loses his patience with them and shuts down the X Files. It later becomes clear that he does that on his orders from someone else (Erlenmeyer Flask) - what is not clear is who exactly he works for. However, he reopens the X Files when Scully is abducted telling Mulder that the continued investigations of the X Files is what "they" fear most (Ascension).
He seems to be straddling a fence-a very delicate one - between his superiors and the agents. While trying to contain their independence, he often helps them through unofficial channels. Mulder and Scully are suspicious of him, but sometimes he has been the only one in the FBI they could turn to in times of trouble. His actions on many occasions convey the feeling that he is deeply concerned about Mulder and Scully. He offers to join the CSM if he gives him the cure for Scully's cancer (Zero Sum) and when Mulder goes missing in Triangle, he puts his job on the line to go out and search for the missing agent.
Whether he actually made the "deal with the Devil" (CSM) is not clear but we do see him threatening the CSM after that. He seems to have a strange relationship with Alex Krycek, the Consortium's lackey who later turns against them and becomes a free agent. Krycek once beat him up to recover for the Consortium the DAT tape containing details of the governments covert operations. Skinner returns the favor a while later punching him and then handcuffing him to a balcony. However now, Krycek has the upper hand. Skinner has small nanobot machines inside him that are controlled by Krycek. If Krycek chooses to do so he can flick a switch and kill Skinner instantly. Krycek now uses this power over Skinner to obtain information that he needs from him (Biogenesis).
Thus he still cannot be truly trusted though he has put himself in harm's way to help the agents. They do their part as well, clearing his name after he is framed for murder.
We learn a little bit about Mulder over the years. He joined the marines at 18, gets sent to Vietnam and in a moment that has affected him deeply, kills a 10 year old boy shooting him in the head from a distance of 10 yards. He later gets ambushed and shot and then has a near death experience. His time in Vietnam caused him to lose faith in his ideals. He returns, puts his military career behind him and joins the FBI where he swiftly climbs up the hierarchy. He is married to Sharon and they were at the point of a divorce (Avatar).