Some characters first appear with most or all of their identifying traits withheld from the other characters and the audience. They likely won't be greeted by other characters as friend/lover/family/colleague/boss. Their faces and forms are often hidden in shadow or actual disguise. Their very presence may go unnoticed by some or all of the other characters. Yet they keep appearing: the author repeatedly mentions them in the text; the camera focuses on them and follows their actions. The audience (and perhaps one or more of the characters) is wondering, "Who are you?" That's the point.
Withholding information in this fashion piques the audience's curiosity. Without it, the audience can't form solid expectations about the character. Part of the plot development may also be in doubt, or else solving the mystery of this character becomes the plot. Or one of several. The audience and the other characters may be let in on the secret in tandem, or the audience may learn the truth first and the characters only find out later. Then again, some secrets are never revealed.
Mysterious Strangers can be a source of surprise for both other characters and the audience, often by displaying an unexpected talent or triggering a plot twist. They may be a source of information via an anonymous phone call or a written note. They may even secretly provide the heroes with material resources. They may come forward at a key moment, throwing the villain off guard or giving the hero a much-needed respite.
The precise nature of their secret(s) will determine their role in the story. Subtropes to this include:
- Angel Unaware - A mysterious character is implied/revealed to be an angel or other supernatural being.
- Anonymous Benefactor - A person who provides a gift (often much needed and/or much-desired) to someone else while concealing their identity.
- The Drifter - A wandering stranger who enters a story by coming to a particular town in his travels.
- Enigmatic Minion - A non-Big Bad antagonist whose agenda and motivations are ambiguous, even to the point of assisting the heroes.
- Mysterious Backer - A powerful benefactor of the heroes who helps out for their own reasons, usually by providing information or material assistance.
- Mysterious Employer - A character who hides in the background and employs or somehow directs the heroes, villains or both.
- Mysterious Parent - A parent who abandons their kid(s) and whose absence is important to other events of the story.
- Mysterious Protector - A secretive figure who appears in a moment of need, aids the hero, and then vanishes again; the aid is often overt, such as repelling a physical attack or directly offering a word of advice or encouragement.
- Mysterious Waif - A child who has suffered hardship or loss or is otherwise rendered helpless. Often orphaned or otherwise cut off from family or some other caretaker.
- Mysterious Watcher - An unknown person seen watching the protagonists, usually from the background.
- Mysterious Woman - A woman, usually sexy or otherwise attractive, who knows more than she reveals.
- The Omniscient Council of Vagueness - A group of people with hidden goals, generally depicted as powerful and all-knowing.
- The Spook - Someone whose lack of information regarding the person's identity makes him mysterious.
- Van of GUN×SWORD, to the point that by the end of the series we still have no idea where he came from. He generally just walks into a town, solves their problem with his giant robot, and walks out.
- Vash the Stampede of Trigun is a bit darker version of this. No one knows who he is or where he comes from, and they are terrified of him—which is unfortunate, because he's really not a bad person.
- In Sailor Moon Tuxedo Mask appears, and leaves Usagi wondering about his identity, and whether he is friend or foe. He's a friend, and they fall in love.
- In Ceres, Celestial Legend Aya is rescued by a mysterious young man. He helps her out several more times as she is running from the Yagami family, and then, they fall in love.
- Xellos from Slayers Next onwards kind of falls into this. We know he's a monster/demon, but he seems to be an ally to Lina rather than acting against them. He has a happy-go-lucky attitude with Eyes Always Shut. But if he ever opens his eyes...
- The original Scarlet Spider was revealed this way with Ben Reilly gradually on his way to meet Spider-Man during the lead-up to The Clone Saga. Strangely enough, since the promotion for this event was so massive, fans knew exactly who he was even though the issues leading up to The Reveal tried to play it off like a surprise
- In Breaking Boundaries, a Town of Salem fanfic, the person who visited mjfan on Night 19 is a good example of this trope. We literally never see what they actually look like. Ever! They are the other disguiser who is constantly in disguise. They don't like being a 'townie' so they rewrote their job to be replacing mafia members during the day when they are tired. This could possibly explain why the mafia is so much more organized then the town.
- In the Magical Girl Crossover Shattered Skies: The Morning Lights, Chibi-Chibi, Cure Echo, Alicia Testarossa, and Nagisa Momoe each appear in the wrong world, leading survivors of the titular apocalypse through PortalDoors to a Place Beyond Time. The four of them are later revealed to be alternating forms of the same being, called "The Stranger", who is an embodiment of those erased from the flow of normal time by the villains' massive disruptions of time and space.
- Cira from A Brother's Price is this when she first turns up. The spoilered name is not her real name.
- Mr. A. H from The Night Circus. We learn almost nothing about him other than 1) He wears grey suits and, 2) He's eerily good at avoiding death and making it look coincidental. Even Widget's interview of him at the end reveals very little about his history, though much more about his philosophy.
- Marco names himself "Alister" after his instructor, though it's implied the gray suit has many names.
- The aptly-named character Anonemuss in The Avatar Chronicles. All we know for sure about him is that he was exiled for having committed some unknown act of violence (which on New Earth can be as minor as slapping someone), and that he has an "ends justify the means" mentality. His real identity isn't known, and even his game avatar, which is all we see of him, is mysterious (in Epic, it's a dark elf, which is a strange choice since his character wouldn't be allowed in the game's cities). By the end of the trilogy, the other characters, and the reader, trust him, but we still have no idea who he is or what his motivations are.
- Strider first appears in The Fellowship of the Ring as a grungy, creepy, weatherbeaten stranger, cloaked with his face hidden, watching the hobbits from a shadowy corner. It's ambiguous whose side he's on or what he wants, the innkeeper doesn't trust him at all, and it rapidly becomes alarming how much he knows about Frodo's secret business. Frodo can only trust him on a leap of faith. He becomes a main character of the Fellowship, an invaluable ally.
- Mr. Rabbit in Rainbows End. He's hired by the international team attempting to track down the "You Gotta Believe Me" virus, but it's clear from the beginning that he has an agenda of his own. After he makes contact with Robert Gu, Robert actually begins referring to him as Mysterious Stranger.
- The Cosmere: Hoid is a mysterious worldhopper who appears at least in the background in every book, including ones set centuries apart. He often has a small but important role in events, giving a few words at the right moment to push the main characters in the right direction. Other worldhoppers, universally, find him an annoyance at best. In The Stormlight Archive he is "the King's Wit" and takes a much more active role, but it's also clear that he enjoys being aloof and mysterious more than is probably healthy.
Kaladin: Wit never gives me answers. At least not straight ones.
Zahel: That's because Wit is an asshole.
- Midway through the first season of Once Upon a Time, a stranger rides into town on a motorcycle. For several episodes we know next to nothing about him, not even his name. He eventually identifies himself as August Booth, and for some reason he knows about the Fairy Tale world. Turns out he's actually from it. He's really Pinocchio.
- Angel in Buffy the Vampire Slayer, for the first series or so at least, mysteriously follows her and advises her. It's several episodes before she even finds out he's a vampire. Later on, he occasionally fights against her, too.
- Babylon 5: The first time a Ranger shows up on the station, we have nothing to indicate who he is or what he wants until he gives Garibaldi a message from Jeffrey Sinclair.
- In The Magnus Archives Sasha, one of the archivist's assistants, meets "Michael", a man (or thing) who engages her in Cryptic Conversation about her colleagues, whose names it [sic] somehow knows. Later in the episode it appears to become a Mysterious Protector when it removes one of the Pest Controller Jane Prentiss's silver worms from Sasha's body which would otherwise have killed her, or worse.
- Exaggerated in Ride the Cyclone, which opens with half a dozen high school choir members dying in a freak roller coaster accident. Five arrive in limbo in relatively good condition (considering the fact that they've died), but the sixth shows up with no head and no memories of her life on Earth. "Jane Doe," as she's christened, is totally unknown to the other students (none of them can remember her), to narrator (he didn't read her fortune before the accident, so he doesn't know who she is), and to herself.
Jane Doe: "Jane Doe" is what the coroner said.
They found my body, not my head.
No parents came, and so they never learned my name,
or who I used to be. My life, an unsolved mystery.
- The perk "Mysterious Stranger" causes a magnum-toting badass in a Nice Hat and trenchcoat (named simply Mysterious Stranger) to appear at random and lend you aid.
- Miss Fortune also counts, although she's pretty much just a female NPC version of Mysterious Stranger with different effects.
- In Fallout 4, your companion Nick Valentine is tracking the Mysterious Stranger under the assumption that he's a possibly-immortal serial killer using stealth tech.
- Fallout: New Vegas has the Lonesome Drifter, who is the Mysterious Stranger's estranged son and can give you a copy of his father's gun. Though it's sadly not as powerful as the genuine article, it does play his theme music whenever you draw it.
- Fallout: Nuka Break has a Brick Joke involving the Mysterious Stranger walking through Eastwood when a gust of wind blows his hat off. Twig picks it up and returns it to him, jokingly saying "now you owe me". The Stranger, apparently either Sarcasm-Blind or just that honour-bound, returns the favour several episodes later by "rescuing" Twig's group from a single measly Radroach, which he empties his entire magnum into. He then declares that he and Twig are "even" and vanishes without a trace. Very much a waste of the Stranger's massive damage output, which is what makes it funny.
- The Master of Whispers in Guild Wars. Is first introduced talking to some other conspirators only, with lots of information kept secret. After a few missions, you find out much more about him.
- In StarCraft: Brood War Samir Duran at first presents himself as a Terran rebelling against the Terran Domion and allies himself with UED. Later on he is shown to working with Kerrigan and the Protoss. The character has fueled many rumors.
- Starcraft II gives him a concrete motive and identity, but his background and potential is left unknown. He is Amon's high priest. He spent millennia planning for the return of the Fallen Xel'Naga, who seeks to turn off the galaxy. It's unknown where the insane drive to help such a monster comes from, but all those years may have eroded his mind. Duran was impaled by Kerrigan, but it's unknown if he was using a meat puppet or if he really is dead. In Legacy of the Void, he is a Xel'naga who served Amon and was return back into the Void when his physical form was killed, where he was in charge of being warden to Ouros, before being slain for real.
- The man in black from the "I Know You" questline in Red Dead Redemption. He knows about things he couldn't possibly have been there to see, some of his dialogue implies that he may be God, Satan, or possibly The Grim Reaper, and your last meeting with him takes place on the hill where John and Abigail are later buried.
- Referenced in Knights of the Old Republic when Ajuur the Hutt makes this your professional name in the duel arena on Taris. The announcer describes you as having no past and no name. Foreshadowing much?
- The Mysterious Stranger in AdventureQuest, before he is given an identity, dresses in a dark gray, face-obscuring cloak. He makes enigmatic comments about a far-off plan years before he gains a significant role in the overall plot. When his storyline is over, it turns out there are more Mysterious Strangers waiting in the wings.
- The Framing Device of Tales from the Borderlands involves the two main protagonists captured by a masked man who is not named in-story but is referred to as "The Stranger" in the credits, who forces them at gunpoint to retell the events of the game to him. In the final chapter, the Stranger's identity is revealed. It's Loader Bot.
- The Strangers from Eternal Card Game are a mysterious group that get each others' powers, making them very dangerous in large groups.
- Subverted in Dragon Mango. An unknown cloaked person urges the three girls to give up being Lolita Knights for a greater challenge. Cupcake rejects this, and the stranger reveals she's Cupcake's mother, Chocolate Explosion.
- The Muse of Time from Girl Genius. Almost nothing is known about her and she has only shown up twice in the series, once on the fourth page but this eerie early introduction gives the readers no aid in learning about her.
- The main Mysterious Stranger in The Letters Of The Devil is the mysterious "L", who has sent letters to a number of strangers revealing deadly secrets. It is suggested that "L" may be the series' other Mysterious Stranger, a bike courier named Gilly Telles, who delivers several of the L Letters.
- In The Avengers: Earth's Mightiest Heroes!, T'Challa/Black Panther spends several episodes lurking in the shadows and watching the Avengers from afar, before revealing his presence to them.