The story involves a mystery where someone has committed a crime or misdemeanor of some sort, and neither the protagonists nor the audience are supposed to know who the guilty party is. However, The Law of Conservation of Detail and the rules of Fair-Play Whodunnit state that the culprit must be a character who appears in the story before The Reveal. It can't be someone the audience has never seen before, and if the mystery is a big part of the plot, it can't really be a minor background character either. It has to be someone important.
In Fair Play Whodunnits and many other types of Mystery Fiction, the writers usually introduce several potential suspects to the crime, and in the end one of them is found to be guilty, while the others turn out to be mere red herrings. However, in some pieces of fiction (typically ones where the mystery isn't the main driving force of the plot), there are no red herrings, and the audience can rather easily deduce the culprit, since they're the only possible major character who could have done it. Either there are no other significant characters among the suspects, or all the other major characters can be ruled out because they're the protagonists, series regulars (in the case of serial media), or other types of characters that aren't typically used as a culprit, such as kids or animals.
Of course, even if the audience can guess who did it, it isn't as easy for the protagonists to solve the mystery, since for them the guilty party could be any minor character, or even someone who doesn't appear in the story at all. It's only the audience who can rule these people out.
If the writers don't care about the rules mentioned above, they can make the culprit turn out to be some completely unexpected minor character, or even someone we've never met before, but these kind of mysteries tend to be much rarer (and more unsatisfying) than the ones that follow the rules.
This trope can overlap with Chekhov's Gunman, if the guilty character doesn't seem to have any proper function in the story before The Reveal. Contrast this trope with Narrowed It Down to the Guy I Recognize, where the viewers can guess the culprit not because he's the only suspicious major character, but because he's the only one played by a major actor.
Warning: the examples below contain spoilers... Though, perhaps, rather obvious ones.
- Attack on Titan: The female Titan is Annie Leonhart. Armin figures out that she's another human-shifting Titan almost as soon as she appears, but her true identity is a mystery for most of the first season. What makes it obvious is that we know from Eren Yeager's transformation that the Titan form shares certain features with their normal human form (such as hair color). The female Titan is blonde, and there are only two blonde women of note on the show -Krista and Annie. Krista was shown to be elsewhere during the female Titan's attack, so that just leaves Annie.
- In Die Another Day, we find out there's a mole inside the MI6 who has, among other things, informed the bad guys who Bond is. Now, obviously the mole can't be Bond himself, nor M or Q or Moneypenny, as they are all mainstays of the franchise. Besides them, there is only one other major MI6 character in the movie, who — surprise, surprise! — does turn out to be the mole.
- In Captain America: The Winter Soldier, Steve Rogers sets out to discover who has ordered the attacks on both him and Nick Fury. Only one plausible suspect is shown, Alexander Pierce, especially after he makes several vaguely ominous statements like "to build a better world sometimes mean tearing the old one down". Perhaps in awareness of this, the film makes little attempt to disguise Pierce as the culprit and it only takes five minutes after he and Steve meet for him to become a visible antagonist. The only reason that savvy viewers might be surprised is because Robert Redford is not known for playing bad guys.
- During the second half of Star Trek VI: The Undiscovered Country it turns out there's a traitor among the Enterprise crew, but no one knows who it is. Since it's obvious they're not gonna make any of the series regulars have a FaceHeel Turn after three TV seasons and five movies, there is only one major character available. Yeah, you guessed it, it's Valeris. This might have been a more interesting had the character been Saavik from Star Trek II: The Wrath of Khan, who the traitor was written in as a replacement.
- In Throw Momma from the Train, When Owen asks Larry why he thought Owen's mystery story was too obvious, Larry explains that it is a three page story with two characters, one of whom dies on page 2.
- When writing the Lord Peter Wimsey book Five Red Herrings, Dorothy L. Sayers said that people had complained that her previous book Strong Poison had only had one possible suspect for the murderer — as had two of her previous books, Unnatural Death and The Documents in the Case. So she wrote Five Red Herrings with the view that if people wanted to play whodunnit rather than howdunnit, she was prepared to indulge them this one time.
- The Scarlet Letter has two major male characters — Hester Prynne's long-lost husband who only recently returned, and the Reverend Arthur Dimmesdale. The reveal of whom she committed adultery with is not terribly shocking.
- Harry Potter: "RAB" is Regulus Black, Sirius' dead brother. He's a fairly obscure character who'd only been mentioned in passing a couple of times prior to the reveal, so it probably would have been hard to figure out... Were it not for the fact that there are no other characters in the series with those initials. Spoiled by the Format was also an issue, as foreign versions of the books that changed the character's name had to change the initials accordingly.
- In Wilkie Collins's "My Lady's Money," Felix Sweetsir—he is the only person with a motive to steal the money.
- In "The Pliable Animal" by Harry Harrison, a prince is murdered on a planet with the strictest possible Thou Shalt Not Kill policy... for everyone but the king, whose duties include performing the occasional Mercy Kill on trapped dangerous predators.
- In the Doctor Who story The Deadly Assassin, the President of the Time Lords is assassinated, and the Doctor framed for the crime. The only suspect who gets any significant amount of screen time is Chancellor Goth, one of the candidates-in-waiting for the Presidency, and he turns out to be the real assassin.
- Towards the tail end of Torchwood: Miracle Day we're told there's another evil Mole in the CIA other than the Obviously Evil, Fat Bastard that was outed earlier. There are three CIA operatives that aren't part of the main cast. One is Da Chief and working pretty diligently with Torchwood and wants the case solved and the mole outed. The other is developing the software to find the mole. That leaves the last one, who was featured prominently in the first episode, who we had just been reminded exists the episode prior, and who just had her wardrobe change up to low cut cleavage showing dresses. To be fair though, the audience is let in on it before the heroes, who were busy juggling the Idiot Ball the entire season.
- Season 3 of The Flash (2014) introduces a new masked villain, "Dr." Alchemy. It also introduces one new supporting character: Julian Albert, an unsympathetic crime-scene investigator who seems to be obsessed with metahumans. Guess who Dr. Alchemy turns out to be. However, in a twist, he doesn't even realize he is Dr. Alchemy, and is Good All Along.
- Subverted in Sleuth (as well as its movie adaptations): the first half of the story features only two characters, and when one of them disappears under suspicious circumstances, it would seem the other one has killed him. But it turns out he never disappeared at all, and the policeman investigating the case is actually the "missing" character in disguise.
- Fire Emblem: The Tellius games have a real problem with this.
- Fire Emblem: Path of Radiance has a subplot about a spy in Ike's army, who is eventually revealed to be Nasir. This would probably be a lot harder to see coming if he didn't meet the heroes right around the time Ashnard mentions getting a spy into Ike's group and then continue to follow them around long after having fulfilled his original deal with them for no clearly stated reason. Genre Savvy players might also be able to pick him out just based on the fact that he's not a playable character at that point in the game, so nothing will be lost gameplay-wise if he betrays you.
- Zelgius is the Black Knight. He's well known as a master swordsman and one of the greatest generals in the world, he's the right build to fit in the knight's very large armor, he uses the General/Marshall class (which the Black Knight's unique class is a modification of) and even shares some of the Knight's otherwise unique animations in cutscenes. Like Nasir above, he isn't a playable character. All of this by itself isn't necessarily damning, but when you combine it with the fact that there's no other viable candidates, it becomes really obvious.
- Bertram is Renning. His helmet doesn't cover his mouth, and there's not a lot of other characters with green goatees.
- The Mole in Persona 5 is very easy to guess simply from the fact that they have a Confidant that automatically ranks up during the story, and because they're the one party member who never got much attention in pre-release marketing.
- The Mega Man Battle Network series often falls into this trope. Whoever is behind the cyber-attack of the week can easily be narrowed down to the NPC with the unique portrait. The Netopia arc in Mega Man Battle Network 2 is especially guilty of this, as Princess Pride fakes her own death to throw off suspicion but is still the only NPC in the castle with a unique portrait and a named Net-Navi you haven't fought yet.
- 'Star Wars Jedi: Fallen Order: The Second Sister is Trilla Suduri, Cere's former padawan. Despite her mask and armor, it's pretty obvious the Second Sister is a human woman and former Jedi, and other than Cere herself, Trilla is the only one to be mentioned in any significant capacity. Her supposed death is also described pretty nebulously prior to The Reveal.
- Ace Attorney cases with a small pool of suspects often end up falling into this. While this usually only happens with early cases, where the killer's identity is never meant as a twist to begin with, later cases can end up like this too.
- Case 4 of the second game plays with this. An assassin committed the murder, so you're looking for the one who hired him. Besides the assassin and the defendant, Adrian Andrews is the only non-recurring character in the case. Sure enough, when put on the stand her story has numerous holes. The twist is your client is guilty as sin, Adrian is innocent. The reason she looks so suspicious is that she tried to frame the guilty party.
- Case 4 of Investigations 2 is a classic example, there are only three people involved in the case who aren't recurring characters, one of them being an old lady who almost certainly didn't have the strength to commit the crime, another being the old lady's granddaughter, and the third being extremely Obviously Evil and adamant about pinning the crime on Kay Faraday. The game does try and set up Justine Courtney as a plausible suspect briefly, but she quickly points out that the evidence shows there's no way she could've done it. Sure enough, the Obviously Evil character did it. And both the old lady and her granddaughter were accomplices via his bullying and blackmail. Though the real twist is just how far the culprit's villainy extends.
- This trope is used frequently in various versions of Scooby-Doo. Subverted in one episode of What's New, Scooby-Doo?, though: the crook in "It's All Greek to Scooby" turns out to be some random person the gang has never met beforehand. Velma naturally is rather displeased and keeps insisting her theory about who was the monster was at least plausible.
- In the My Little Pony: Friendship Is Magic episode "Rarity Investigates", Rainbow Dash gets framed for a petty act of treachery so that she won't break the long distance speed record. There's only one non-recurring character in this episode: Wind Rider, the holder of said record.
- Batman: Under the Red Hood opens with the Joker killing Jason Todd and then introduces the Red Hood in the next scene (which takes place years later). Batman spends the middle portion of the film trying to figure out who the Red Hood is, but literally all of the film's major characters other than the Red Hood are shown fighting and/or talking to the Red Hood, so even a viewer who's not familiar with the comics will probably figure out that the Red Hood is a resurrected Jason Todd before Batman does.