But when Dana Scully was kidnapped, he reopened them, telling Mulder, "That's what they fear the most" -- without clarifying who "they" are.
It's never clear whose side Skinner is on, or whose orders he takes, but his efforts on behalf of Mulder and Scully "through unofficial channels" have made it plain that he is deeply concerned about his independent-minded agents and their work.
This profile in the computer game says "No children."
From the fourth season, he lives on the 17th floor of Viva Tower, which is marked as no 828 of whatever street it is on. This is in Crystal City, Virginia. He tells Cancerman he has just moved in. Barely six months earlier ("Avatar") he had also just moved house, after his divorce, but it is not known where to.
Age?
His age or birth date is never given in the show. However, he went to Vietnam aged 18 and did at least a partial tour of duty, so must have enlisted in 1972 or earlier, placing his birthdate as no later than 1954.
If you want to accept the computer game as canon, his date of birth is June 3rd 1952
Physical Stuff
The computer game states that his height is 6 feet 2 inches and his weight is 200 lbs.
His eye colour is Brown
In Vietnam
In "One Breath", Skinner tells Mulder how, on his eighteenth birthday, he volunteered to join the Marine Corps in Vietnam. "I did it out of blind faith. I did it because I believed that it was the right thing to do. I don't know - maybe I still do." Three months in to his tour a ten-year old boy covered in grenades walked into the camp, and he blew his head off ."I lost my faith," he says. "Not in my country, or in myself, but in everything. There was just no point to anything any more." Then he was ambushed while on a patrol, and looked down from outside his body, feeling peaceful and afraid. He awoke in a Saigon hospital two weeks later. "I'm afraid to look any further beyond that experience," he says.
In "Avatar" he expands on this, saying he saw an old woman in his dreams several times in Vietnam. He put it down to a drug-induced hallucination, but she was hard to ignore. In his near-death experience she seemed to be there with him carrying him away from the light, back to his body.
As a nice footnote, there is a PFC (Private - First Class) Walter F. Skinner on the Vietnam Memorial (the Wall). It's on Panel 41E, Line 30, first name on the left. Moreover, PFC Skinner was a Marine.
Background
The computer game says "University of Texas (BA degree)"
Drug-taking
In Vietnem, as he tells Mulder in "Avatar", he tried to numb the pain of the experience by taking drugs. "I was no choir boy," he says. "I inhaled."
Sleep Disorders
For the three months before "Avatar" (set in March 1996), Skinner was being treated at a sleep disorder clinic, suffering from a recurring dream of an old woman who speaks to him in words he can't understand then straddles his chest, smothering him.
Hobbies
"Golf, jogging", says the computer game. In SR 819 we see him boxing (at South Street Gym), and winning. His trained called him "Slugger"
Health
In S.R 819 Skinner is infected with something that puts nanomachines in his blood - things that can be controlled literally by the push of a button. Krycek controls them. At the end of the episode, Krycek chooses to save Skinner's life, but both he and Skinner know that the things are still in his blood, and can be re-activated any time. Thus, from this episode one (until we're informed otherwise), Skinner's blood is a time-bomb, and his illness could resurface any time... presumably.
Attitude to The X-Files
Episodes in which he reveals something of his attitude to Mulder and Scully. Most of these are second season, as this is the complex time, when his loyalties aren't clear. By the fourth season he seems unambigiously a friend of the X-Files team.
 In "Tooms" he tells Mulder how much he admired him while he was in the Academy and how he, like everyone else, had high hopes of him and thinks his talents are wasted on the X-Files.
At this point he seems to be toeing the party line, deferring to Cancerman. However, there is a hint of doubt in his eyes when confronted with Mulder's report. He asks Cancerman if it should be believed.
He's the one who closed the X-Files down in the "Erlenmeyer Flask", though he was acting on the orders of the highest levels of authority.
In "Little Green Men" he is furious with Mulder for running off to Puerto Rico, but seems to be even more furious with Cancerman for threatening Mulder with punishment. He seems to have the attitude that it's fine for him to tell Mulder off, but no-one else can do it, especially someone who doesn't have any official authority
He is probably the "friend in the FBI" that X tells Mulder about in "The Host". However he makes no attempts to protect Mulder from his own weaknesses. When Mulder comes storming into the office shouting, he calmly asks him in and lets him embarass himself in front of the room full of people.
The heart of his attitude seems to be professionalism. While he might sympathise with Mulder sometimes, he can't sympathise with him neglecting his duties or treating a murder case as meaningless. He brusquely reminds Mulder that the X-Files are closed and he is to carry out his new assignments to the best of his ability.
Later, Mulder accuses him of shutting down the X-Files - the very people who could have solved the case. "We all take our orders from somone, Agent Mulder," he says, looking awkward.
"Ascension". When Scully is missing, Skinner tells Mulder to get some rest, saying he is too close to the case. This is just sensible, and doesn't suggest any sinister intentions to keep Mulder away from the truth.
Later, when Mulder tells Skinner about his suspicions of Krycek and Cancerman's involvement in Scully's disappearance, Skinner is at last moved to act. "There's only one thing I can do, Agent Mulder," he says. "As of right now I'm reopening the X-Files. That's what they fear the most."
"One Breath" - Skinner again defies Cancerman. "If you're having trouble sitting on Agent Mulder," Cancerman says, "I'm sure you know we'll have no trouble."
When Mulder demands to know who Cancerman is, Skinner is angry, asking if Mulder intends to kill him. He reminds Mulder that they work for the Dept of Justice, but Mulder stresses that it's justice that he wants. Evidently Mulder persuaded him because he later tells him Cancerman's address.
He reminds Mulder that Scully knew the risks and that everyone in law enforcement accepts these risks. When he gives Mulder Cancerman's address he says he's not risking his life - not any more than normal. "Every life - every day - is a danger. That's just life."
This is also the time that he tells Mulder about his near-death experience, saying that he's afraid to look further than that but Mulder isn't.
"Colony" and "End Game" - In "Colony" we find out that Skinner and Mulder have an "understanding" about the X-Files, which seems to mean that Mulder can run off on cases without asking permission. However, when an agent dies on the case, Skinner's not so understanding, remining Mulder that his own superiors aren't so understanding either.
In "End Game" he responds to Mulder's desperate plea for his trust and help, obtaining a sharp-shooter for the deal at the bridge. When Mulder goes missing he apparently refuses to help Scully, even through "unofficial channels", but he ends up fighting X to obtain the information.
"F Emasculata" - Skinner apparently condones the cover-up of the outbreak - but then so does Scully. Mulder asks him where he stands in this. "I stand right on the line that you keep crossing," Skinner replies. As Mulder and Scully turn to go, Skinner calls Mulder back. "I'm saying this as friend," he says. "This is just the beginning. Watch your back."
"Anasazi" trilogy - Although assaulted by Mulder, Skinner apparently finds no pleasure in the disciplinary panel.
After Mulder's "death", Skinner tells Scully he intends to make sure the man who killed him are punished. "With all due respect, sir," she says. "I think you over-estimate your position in the chain of command." He retaliates nicely when she demands that he investigate the death of Kenneth Soona. "Miss Scully, I think you underestimate the duties and responsibilities of my position as Assistant Director," he says. However, on this occasion, his coldness to her is probably explained by the fact that Cancerman is hiding next door listening.
When Scully is warned that someone will come to kill her, she immediately assumes that Skinner is the one as soon as she sees him.
Later, when Skinner reveals that he has the tape, he says that it wouldn't "do us any good" (note the "us") if it just fell back into their hands, and that the tape is the best way of bringing the men to justice.
He finally openly stands up to Cancerman, using the tape as a bargaining counter to get a deal - to get Mulder and Scully reinstated. He says that he'll go state's evidence if they don't honour the deal. "Then they'll have to kill me too."
 "Nisei" - Skinner makes clear that he wants no involvement with this case and Mulder's on his own.
 "Piper Maru" and "Apocrypha" - Skinner gets shot over the issue of Melissa's murder. Once again, it is justice he is concerned with. He's not concerned with all the alien stuff that Mulder's worrying about, but is concerned that the killers of a young innocent woman aren't being punished. "You take your orders like those below you," a sinister man threatens before the shooting "It helps to remember these things if a man looks forward to his career, to his plans for the future."
At the end of "Apocrypha", Mulder thanks him for what he did - for sticking up to "them" even at the cost of getting shot. "I think you're proceeding from a mistaken impression," Skinner says. "What I did I did because it was my job." "From what I understand you put your job and your life on the line for Scully," Mulder says. Skinner says it was not his crusade. He simply saw a case in which a woman was murdered and he mistakenly thought the killer could be brought to justice.
 "Avatar" - While this is a Skinner episode, it doesn't cover much of his attitude to the X-Files, except for the fact that his protection of the X-Files is sufficiently worrying for Them to want to get him removed. They use his alleged support for Mulder's wild theories as an excuse to discredit him, saying he has become "enchanted" by Mulder's ideas.
This episode also tells us that Mulder was one of the few people from work that Skinner ever mentioned at home, saying how much he respected him.
 "Talitha Cumi" and "Herrenvolk" - Skinner says there was a time when he could have helped Mulder find Cancerman, but no longer.
In "Herrenvolk" he does some research of his own into the Smith clones, but challenges Scully when she presents her research to a room full of high-up FBI people.
 "Tunguska" and "Terma" - He helps Mulder shelter Krycek, although this is not strictly legal, and beats him up a bit as payback for the beating he took in "Paper Clip."
He is summonsed, along with Mulder and Scully, to the Senate committe, charged with subverting the course of justice. For someone so fixed on justuce normally, Skinner is not well pleased at this, especially as Scully won't tell him much.
After Mulder's return from Russia, he and Scully make plans for the next step in the case. Skinner says he doesn't want to know. If he doesn't know where they're going, he can't have to explain it to the committee o r his superiors.
"Paper Hearts" - Not an X-File / Conpiracy case, but interesting as it shows how Skinner seems to look to Scully to keep Mulder in line. When Mulder breaks every rule going, Skinner seems to expect it of him, but is angry at Scully for not stopping him.
"Memento Mori" - Skinner definitely now feels that "they" are the villains of the piece and the X-Files are the heroes. He implies that Cancerman is the devil, and urges Mulder not to make a deal with him as "they" would own him thereafter. However, he then makes the deal himself, at an unspecified cost.
"Zero Sum" - We see the deal put into operation now. Skinner reluctantly covers up a series of crimes, before realising that he's been tricked and Cancerman has no intention of keeping his end of the bargain. Mulder finds out about the bargain at this point too.
"Redux" - Suspicion falls of Skinner in "Redux" when it is discovered that the man spying on Mulder was making calls to an executive level phone number at the FBI. While Skinner himself is not taken in by Mulder's faked death and, in reality, seems to be working more to uncover the truth, Scully begins to distrust him. There is a nice point when she is being questioned by Blevins, and Skinner is standing in the background in exactly the same position as CSM was in the Pilot episode.
Scully and Skinner confront each other, and Scully accuses him of spying on her - "Is this more dirty work you're doing for the DOD?" she asks, angrily. He says that he is concerned about her. He knows she is lying to the committee about Mulder, and fears that, when the truth is revealed, it will do her nothing but harm. Scully is not convinced, for she is on the point of accusing him publically when she collapses.
In due course, Mulder finds out that Blevins, not Skinner, was the guilty person, and Skinner seems very relieved when Scully goes into remission. He actually smiles!
However, a few fans have been making dark murmurs that Skinner looked a little too relieved when Mulder named Blevins. Whether the man still has something to hide or not (and he's come out on Mulder and Scully's side many times now), the potential is there to be exploited in fanfic should anyone be that way inclined.
In "The Red and the Black", Skinner admits to Mulder that he has begun to believe all the evidence that he (Mulder) has presented. Given all this, he says, belief in alien abduction is a whole lot more plausible than the government cover-up theory that Mulder is now as passionate about as he used to be about aliens.
("Over the past five years," he says to Mulder, "I've doubted you, only to be persuaded by the power of your belief in extraterrestrial phenomena, and I'm doubting you now, not because of that belief, but... Because extraterrestrial phenomena is, frankly, the more plausible explanation.")
In "Folie a Deux", Mulder believes (whether rightly or wrongly) that Skinner is angry with him, and is giving him a boring dead-end assigment. He calls himself "Monster Boy", and says that Skinner is using him to deal with anything weird that no-one else can be bothered to look into. At the end of that episode, Skinner seems pretty insistent that Scully admit to seeing the monster too, not taking her "it was dark" cop-out.
"The End" starts with Skinner in Mulder's office, asking him how he sees his future. When Mulder is prepared to risk the X-Files in order to get proof, he cautions him, as if he is reluctant to see the X-Files go down, or else because he knows how important it is to Mulder (after all, it was at the start of the same episode that Mulder told him that the X-Files are still his whole life and his whole future) and knows how destroyed he would be if it all goes wrong.
"Triangle": He helps Scully find Mulder, even when everyone else refuses. He covers it up by shouting at her when CSM appears. He also gets a kiss from Scully, in gratitude, and, at the end, turns up at Mulder's hospital room bearing flowers... (This bit really happened. The Scully scenes in the FBI may have been part of Mulder's dream, too, meaning he dreams a Scully-Skinner kiss. Hmm...)
S.R 819: Skinner states his position: "Every minute of every day we choose. Who we are, who we forgive, who we defend and protect. We choose a side or to walk the line. To play the middle. To straddle the fence between what is and what should be. This was the course I chose. Trying to find a delicate balance of interest that can never exist. Choosing by not choosing. Defending a center which cannot hold."
Later in the same episode, when he thinks he's about to die, he apologises to Scully, saying "I can see now I always played it safe. I wouldn’t take sides, wouldn’t let you and Mulder pull me in." She assures him that he's been their ally many times, but he says "not the kind of ally that I could have been."
At the end of the episode, however, he seems to have found out that Krycek is the one who chose to save his life, and is acting as if he owes Krycek because of this. He calls off Mulder's investigation of what happened, and asks Krycek "what do you want from me?" ("All in good time", Krycek replies.)
Monday - He is very concerned about Mulder and Scully being caught in a bank robbery, desperate to get involved in rescuing them. He dies when the bank explodes - though luckily this isn't the version of reality that we end up with. Like so much in season six, it didn't really happen