Nothing Is Scarier than someone you know nothing about. He has nothing to identify him, nothing that even states that he is real. If this person simply disappears, you are terrified because you don't know if he is watching. Sometimes he is established as a myth, a ghost story to scare people. This person lacks not only a past, but also a present and a future. He just is, and you don't know why.
The Spook is someone whose identity is a paradox, they exist but don't exist at the same time. No one can prove that he is there except by showing someone the warm body, if you even have it. If a name is given it is likely to be Shrouded in Myth, you can't be certain of anything you hear about them but you don't want to underestimate them all the same.
In some cases, this may overlap with a Legacy Character: the myth is more important than the person.
All There in the Manual may have extra information that is confirmed by Word of God, but that doesn't count: this is about how this character is perceived by others. What matters is how the characters in the story behave in regard to the stranger. If the people in the story know all about the character, then it does not qualify as The Spook even if the audience knows nothing.
This kind of character is often seen as either an Enigmatic Minion, or the guy the villains (or even the heroes) bring in to do the job right... even if they don't know what they're dealing with. Also frequently The Nondescript.
Compare The Faceless, but The Faceless is only important based around the eventual reveal (whether or not they actually have a face). The Spook will usually remain an enigma even if caught and defeated. Also compare The Men in Black, who are often mooks with this characteristic, and The Cowl, when a hero tries to be this to the villains. The Chessmaster can sometimes be a Spook, and is often the reason why the character is popular. Bonus points if he is an Implacable Man. Finally, compare/contrast Unperson, where someone else does this to a person as a form of punishment. See also Outside-Context Villain.
Also not to be confused with The Ghost, The Shadow or especially The Spock. Nor to be confused with The Spook.
All members of ACROSS in Excel♥Saga must maintain this secrecy to ignorant citizens, which is why they can only get temp jobs.
Much of the manga is a comical Deconstruction of the trope, noting how difficult it is actually to live without any kind of identifications, steady job or social security. The protagonists end up getting mistaken for illegal immigrants several times over.
L, Mello and Near from Death Note. Justified, as the titular notebook can kill anyone whose name is written in it. Thus, the only people who could challenge the Death Note's user would be those with entirely unknown names.
Light Yagami initially becomes a spook when he gets his Death Note from Ryuk- a mysterious killer with the ability to kill off anyone in the world, location be damned, so long as he knows the real name of his victim. He exacts the power of the Death Note enough that the world notices the sequential killings and dubs the culprit "Kira." That would've been the end of it, had Light not fallen for a trap set out by L by writing down the name "Lind L. Taylor," a man being used as a decoy proxy for the real L. Taking the bait allows L to start hunting him down relentlessly, opening the biggest can of worms he'll ever know.
Johann tries to become one in Monster by destroying all traces of his past. Even without him doing that, his past is still very foggy due to the circumstances of his birth and upbringing. Finding out how he became the way he is is one of the plot's driving forces.
Hazama is this in BlazBlue: Alter Memory. Between attempts to help Noel, Makoto (Hazama's own subordinate, by the by) has been trying to gather information that could link him to the Imperator, and found absolutely jack about him in the NOL database. Whatever justification exists cements to Makoto that Hazama is not to be trusted, and she arranges for a pickup with Sector Seven to get Noel out of Kagutsuchi. It's not a spoiler to say this doesn't work out.
The Octopus in The Spirit. In the entire run of the series, neither the heroes, the villains, nor anyone else ever saw his face or learned who he was. In his first appearance, he ran through fire to avoid revealing his identity.
Mr. Nobody in Spider-Girl. Even when he's apprehended by the police, it's found that he literally has no fingerprints and that what was thought to be a full facial mask is apparently his actual face.
The Boys: The unnamed Vought-American exec is this. The Homelander, a Kryptonian-class superhuman, is more than a little scared of this apparently ordinary human; even half-jokingly theorizing that the executive might be some sort of quintessential corporate lifeform.
Incidentally, this is also what makes Batman so scary to the bad guys. He's essentially the Joker, but wearing a mask and acting against the villains. The difference, that Sal Maroni makes clear, is that Batman is The Fettered. Once they figure out his "one rule" Batman's status as this is diminished. The Joker has absolutely no rules.
Keyser Söze of The Usual Suspects was something similar to this. The nature of the movie made his shadow-ness even more obscure and vague. But even with the things confirmed by the police interrogators, Söze was someone who has never had a confirmed sighting, regarded as a myth, has multiple versions of his backstory and you don't know what is fact or fiction about him.
This is what is often overlooked in The Terminator movies because of the Time Travel and evil robot sci-fi, but the original movie was primarily a person being hunted by someone she knew nothing about, even the reason why she was being hunted.
When you join the Men In Black, you become one of these people.
Jason Bourne is an example of the "made anonymous by support from powerful intelligence agency" variety.
All the field members of Treadstone are made to become this. And with Bourne losing his memory, it shows what happens when one of these people do a Heel-Face Turn.
Chauncey Gardiner in Being There, who grew up as a gardener for a rich old man who had secluded himself and Chance from the outside world. After his master dies, and he has to go out in the world, he becomes a mysterious figure without a past.
In Remo Williams The Adventure Begins, the bad guys refer to Remo as "The Faceless Wonder" because they can find absolutely no information on him at all other than a picture and the fact that he's interfering with their nefarious scheme.
Cobb from Following. The police have no record of his existence, and he tricks his fall guy into dressing and looking just like him. The final shot of the film is Cobb stepping into a bustling street and completely disappearing.
Ernst Stavro Blofeld is this is the James Bond movies. He is one of the few Bond villains to be given no backstory whatsoever, either from his own lips or from a government dossier. This is not the case in the novels the films were based on, however.
Roat, the villain of Wait Until Dark. Roat, incidentally, is not his real name.
The villain of The Hitcher is called a ghost because there is nothing that identifies him.
The Operative: I have neither. Like this facility, I don't exist.
Star Trek Into Darkness portrays Section 31 as this. All of their doings are very hush-hush and under the table, while some of its operatives, like John Harrison/Khan, are given false identities to hide their true nature.
In No Country for Old Men, practically nothing is known about the relentless assassin, Anton Chigurh. The only thing we know is his name, maybe.
Captain America: The Winter Soldier has the titular villain. Natasha literally calls him a "ghost," and notes that most of the intelligence community doesn't believe he exists. Nobody knew anything about him or who really he was until Steve saw his face and realized that he was Bucky, who didn't recognize him. From there, he was able to deduce how Bucky ended up as the Winter Soldier.
The Demon Headmaster is the unassuming head of a small comprehensive school who just happens to have mind-control powers and an obsession with bringing the world into perfect, emotionless order. We know absolutely nothing about how he got these abilities, where he comes from, or what his real name is - everyone just calls him "the Headmaster". Even in "Takes Over", when his amnesiac clone's main goal is simply to learn what his name is, we still don't learn it ourselves.
Julian from The Boy Next Door. He simply appears in the seemingly abandoned house next door to Randy and she knows very little about him.
Burn Notice invokes this several times. One of Michael's plans was based around infiltrating a family of gun dealers by playing off of one of the brothers. By skillfully stealing their supplies, he then simply dropped off the face of the Earth. He explained that nothing scares you more then a spook, someone you know nothing about. He could have been FBI, a rival group, the Mafia; the point was they didn't know who he was and if he was going to return. The group was so spooked, they left the city.
Chuck Finley is everyone, and no one. He's simply Chuck Finley.
Pick pretty much any member of the shadow government on The X-Files (particularly Deepthroat and X). The Cigarette smoking man is the only one that we ever learn anything about and what we know is vague and occasionally contradictory.
This is how Moriarty is treated in Sherlock, though he gets to meet our heroes face-to-face in the last episode. Twice.
The Silence from Doctor Who. Once you look away from them, you forget that they even exist. They often use this ability to plant commands in people's subconscious.
The Doctor was this for much of the original series' run, as very little about his origins or his true identity were ever dwelt upon. On several occasions he's taken steps to remove himself from the historic record. The "Cartmel Masterplan" was supposed to restore this aspect by gradually Ret Conning what was already revealed about the Doctor to be a lie and that he was actually a more powerful figure than he appears, but it was derailed by the show's cancellation.
The Greek of The Wire. By the end of the series, we only know one thing about him: he's not really Greek.
Thanks to the Arcane Fate, the Sidereals of Exalted turn this Up to Eleven. Not only are they completely unknown to the vast majority of Creation's inhabitants, but within a week or two of meeting them, you will forget them. And any records of their existence will be lost, accidentally destroyed, vanish mysteriously, or otherwise be rendered useless. A Sidereal could murder your parents right in front of you, and in a month you wouldn't be able to pick them out of a lineup or even remember the incident clearly—were they even murdered at all, or was it a freak accident of some kind?
In Warhammer40000, the Dark Angels Space Marines are served by the indigenous population of The Rock, the Watchers In The Dark. They are covered head to toe in robes, and have never let anyone see what's underneath. Ever. Games Workshop's official models for them don't have heads, just hollow hoods. Librarians can't probe them because they resist all psychic powers and forms of Warp contact. Are they aliens? Undead spirits of the Fallen forced to serve penance? No one knows.
The GURPS Advantage "Zeroed", created for GURPS Cyberpunk and popular in conspiracy games, makes a PC the Spook.
In Halo, operatives from the Office of Naval Intelligence get this trope name as a nickname for the exact reasons mentioned here. Reflecting this, the Men In Black-esque organization itself is more commonly referred to as ONI, a wordplay on a certain youkai type.
In fact, for the majority of the Human-Covenant War, the Master Chief himself and his fellow Spartans were this to most people on both sides.
Yume Nikki: There's no explanation for anything whatsoever, no Back Story, no dialogue, nothing. Madotsuki, the main (and only) character is a Hikikomori in a flat. She keeps a very trippy dream diary. The building she lives in is apparently only as wide as her flat. Who is she? What is she? How did she come to be there? Where does she get those ideas and dreams from? Is one of Madotsuki's dream characters based on a piano teacher? Is another based on a friend? Is said friend dead? Is another based on a horribly mutilated and/or bullied girl? Is the highly sexual Kyuukyuu-kun based on rape or something? She's basically Mesme without the Psychic Powers. It is safe to say that this singular character has spawned EpilepticForests on her own. The ending of the game itself leaves all questions unanswered, leaving things open to interpretation: Madotsuki commits suicide by jumping off her balcony by using stairs that were never there. Your guess is as good as ours.
Even the creator of the game, KIKIYAMA, is this. Is KIKIYAMA male or female? Do the ideas for Yume Nikki come from alchohol, drugs, a very twisted imagination or a completely normal person who just wanted to make a scary game?
Even after learning in "Meet the Pyro" that The Pyro thinks that everything s/he does is give candy and happiness to other people (which s/he imagines as cupids and other such cute creatures), we still don't know anything about their Back Story. Or anything else.
Albatross and Steven Heck in Alpha Protocol. The dossiers you build on them are about 90% conjecture.
Albatross is the head of an entire organisation of spooks: G22 oppose Halbech and have dirt on nearly everyone, but anything beyond that is pure guesswork. Unless the player gives them away, the Big Bad (and thus Alpha Protocol itself) doesn't know they even exist.
In Phoenix Wright: Ace Attorney - Dual Destinies The villain of the final case turns out to be a Master of Disguise known only as The Phantom, able to perfectly impersonate anyone... to the point where all traces of their original identity are completely gone. We never learn their real name or see their real face: they're always shown wearing a mask of someone else. They feel no emotions except fear of being caught. Their Villainous Breakdown has them freaking out about their "real face" not even existing as they pull off masks of nearly every other character in the game. They collapse into the shadows after removing the final one, so no-one ever finds out what they really look like.
Calisto Yew from Ace Attorney Investigations counts too, as we never find out their real name or backstory, and the two identities we see them in over the course of the game are fake.
Flemeth from Dragon Age. With the revelations (or lack thereof) in Witch Hunt and Dragon Age II, we really know nothing definitive about her at all. She goes mostly by Flemeth but she also has many names, she looks human but this is cast into great doubt by Morrigan, Fenris, and Anders. Her past is Shrouded in Myth, with no clear way to know if it's true. And on top of all of this, her powers and precognition are at a level that aren't seen anywhere else in the series. Her motivations are also completely unknown, but hints are dropped that it is on a scale outside of any other in the franchise.
We may know a great deal about the Slender Man and what he does, but even then there is no way to know his true nature/origin without being taken or worse, especially as he is said to change depending on the mind encountering him. He is everywhere and nowhere. By believing in him you open the door. Attempts to capture or study him almost always go hideously wrong. Marble Hornets gives a possible origin story for him but it's left highly ambiguous if it's true or not. note One character relates a story about how a certain place would tie convicted criminals to trees, stretching them out and letting them die of dehydration before burning the tree with the body attached. His eyes are closed, his mouth is open, and his arms are outstretched.
Combustion Man from Avatar: The Last Airbender . The heroes don't know his name, never heard him speak (the only noises he makes are grunts which aren't even credited by a voice actor), and know nothing about his past. In their first encounter with him, they only escaped by the skin of their teeth. Zuko apparently knows more about him (name, past, etc.) but his portrayal is that of a relentless shadow.
From the Sequel Series, The Legend of Korra, absolutely nothing is known about about the Big Bad, Amon. He gave a backstory but whether it's true or not is up for debate. We don't know what he looks like, what his real name is (if Amon isn't his real name), we don't know what exactly he does to take away peoples' bending or how he even has the ability, we don't know if he's being honest about his intentions or if he has ulterior motives, and we don't know how he's able to resist bloodbending. That is, until the Book One season finale.
The DCAU Joker also has elements of this, especially in Batman: Mask of the Phantasm. We actually see what he was like before he fell in the vat of chemicals: a nameless, voiceless, perpetually smirking mob hitman who might just be creepier than the Monster Clown he turned into. He sparks a good chunk of the plot by killing Andrea Beaumont's father.
The Somerton Man. His corpse was found in 1948 on Somerton beach in Adelaide, Australia. More than 60 years later, his identity or the circumstances than headed to his death are still unknown.