Red Herring Shirt

Basically, when someone looks like a Red Shirt, a Mook, or even just an Innocent Bystander, but turns out to be a Mauve Shirt, or even The Hero or the Villain, with few or no clues beforehand.

Often involves Mook Promotion.

Compare Ascended Extra (where character's promotion in importance happens in an adaptation), Chekhov's Gunman (where a character is introduced in passing, but is deemed unimportant until later), The Dog Was the Mastermind.

Contrast Decoy Protagonist, Disc One Final Boss.

This is a Red Herring trope so all examples will be spoilers

Examples of Red Herring Shirt include:

Anime & Manga

• The Young Conducter / Claire Stanfield / Vino / Rail Tracer of Baccano!. The audience is led to believe that he died in the second episode, and has no reason to suspect otherwise until the ninth episode, despite his heavy involvement in between. To really get the point across, he's one of the 23 characters who appear in the opening credits, but he isn't one of the 17 identified by name. The entire Martillo family appears to be dead after being gunned down by Szilard. A few episodes later, it is revealed that nothing could be further from the truth.
• The first episode of Heartcatch Pretty Cure introduces the Victim of the Week, Erika. Hey, wait a minute, why is she so prominent in the opening credits?
• In the second chapter/movie of Kara no Kyoukai:, a seemingly random student at Shiki's school makes a cryptic statement to her in the hallway. Kokutou soon mentions he was at a party for a student named Lio Shirazumi, who was dropping out of school. Neither the name nor the person are ever mentioned again. You later realize that the random student was Shirazumi and that he is actually the murderer behind the events of both the second and seventh chapters. Somewhat less of a surprise in the movie, because Shirazumi is voiced by Soichiro Hoshi, so you figure he's going to show up again and be important later.
• Mikael from Tenshi ni Narumon. In the first season he was nothing more but a background character, sometimes appearing only at the very end of episode or not appearing at all. In the second season, it was revealed that he is one of the three main individuals important to the plot, then he got promoted to a final villian and at the end, it turned out that he, in fact was the main hero of the show -- confirmed by Word of God.
• Angela from Black Butler appears as a the maid to the Big Bad of Episode 7. She's "beautiful" (which is standard fare) and a domestic servant—that was pretty much it. The Stinger, however, is ominous enough and subsequent episodes make it obvious she's the Big Bad of the season.

Comic Books

• This was a somewhat popular technique in The Golden Age of Comic Books, and was usually Played for Laughs. In one of the earliest Justice Society of America stories, for example, the heroes take down a nationwide crime syndicate whose leader turns out to be an Expy of Newspaper Comic character Casper Milquetoast who had popped up as a bystander in each prior chapter of the story

Film

• The Dark Knight begins with a bank robbery in which disposable thugs kill each other one after another. Finally just two are left, and one is Genre Savvy enough to realize the other guy probably has orders to kill him, so he tries to strike first. Unfortunately for him, this other mook is The Joker, and he was already expecting it and planning to kill the NEXT guy who was coming to kill this guy for him.
• Happens in Bill and Teds Excellent Adventure, when the two robed executioners turn out to be So-crates and Mr. The Kid in disguise, and they rescue Bill and Ted.
• In The Three Musketeers 1993, when D'Artagnan is led to the chopping block, two of the execution personnel turn out to be Porthos and Aramis in disguise, and they're there to rescue him with help from Athos (who takes care of the escape transportation by appropriating the Cardinal's carriage).
• Happened in the Mortal Kombat movie near the end, when Shang Tsung was doing some kind of ceremony with the kidnapped Sonya. Some random "evil" cloaked dudes turn out to be Johnny Cage and Liu Kang in disguise. Cue butt kicking.
• The "two weeks" scene in Total Recall.

Literature

• The Worker from Atlas Shrugged. He is, of course, John Galt.
• Tigerclaw was a random prologue character in the first Warrior Cats book. Fans now know him as The Big Bad.

Live-Action TV

• John Sheppard in the Stargate Atlantis pilot. When the famous O'Neill is flying in a helicopter to meet Weir and company, you pay little attention to his pilot, even when he's given a name. Then he wanders into the Antarctic outpost and activates the control chair by accident, and presto! He's The Hero, ready-made.
• At the end of the first series of Blackadder, two servants standing in the background turn out to be Percy and Baldrick, who proceed to rescue Edmund... almost.
• Jonathan from Buffy the Vampire Slayer starts out as just a generic Sunnydale High Student/Monster Fodder, being used pretty much as scenery. Then he gets a handful of episodes where his insignificance is played up for all it's worth. Then he becomes a major character. Then he's murdered by one of his best friends. Joss Whedon has his obligations to keep up.
• Seska from Star Trek: Voyager. She was originally just a red shirt that appeared in a couple episodes as a member of Voyager's crew. As it turns out, she is the main antagonist of the first two seasons.
• Megan from Mad Men. For over half the season, she's just a secretary/receptionist at Sterling Cooper Draper Pryce, albeit one who was more stylish than the others. By the end of the season, she's Don Draper's fiancee.

Manhwa

• Taken to the extreme in the first chapter of Shin Angyo Onshi. The main character of the series appears only as some henchman of the unknown evil lord of the city. In chapter 2, after it turns out that the supposed hero from chapter 1 was the evil lord all along, the true main character reveals his identity and kicks some ass!

Opera

• In The Gondoliers by Gilbert and Sullivan, the girls who will be the gondoliers' love interests are chosen by a game of blind-man's-buff. Before they are tagged in the game, the female leads of the play have no solos and are not distinguished from the rest of the chorus in any way.

Professional Wrestling

• Often, Professional Wrestling does this, with a Jobber that scores a fluke victory and goes on to make something of himself. See: The 1-2-3 Kid (later knows as X-Pac) and Barry Horowitz in WWE, Bill Goldberg in WCW (who actually started his win streak with his first televised match, despite getting the Jobber treatment at first), Mikey Whipwreck in the original ECW, and Colin Delaney in the WWE Revival thereof. Another popular bit is a fan that somehow gets involved in the action and ends up wrestling; WWE did this bit with Steve Blackman, Zach Gowen, and Santino Marella, as well as having "celebrity guest" Lawrence Taylor get involved this way.

Video Games

• In Final Fantasy VII, one of the Shinra guards in some flashbacks turns out to have been Cloud the whole time.
• Marx from Kirby Super Star, who at first glance appears to be nothing more than a generic resident of pop star. Turns out, he was actually the Big Bad who staged the whole thing, and was manipulating Kirby into awakening Nova so he could use its power to seize control of pop star. Especially since unless you waited at the title screen, he doesn't even appear in the game proper beforehand.
• In Mortal Kombat 9, Johnny Cage is one of the few characters who doesn't die, a total inversion of the character's role in the original trilogy.
• Thor "Lemon Boy" Herring (whose name is probably not related to the trope) in Backyard Sports.
• In Persona 4, the Sorting Algorithm of Evil runs: Mitsuo, a creepy kid who shows up once on your first day of school and appears in the shopping district every now and again, Namatame, a character mentioned in the throwaway news reports near the beginning of the game and sometimes shows up around town, Adachi, Dojima's bumbling Comic Relief of a partner, and Izanami, the gas station attendant who greeted you at the beginning of the game.
• When the Fold take over the nanite-controlled supervillains in Marvel Ultimate Alliance 2, Bullseye kills a large number of Red Shirts with his lethal playing cards. The only one that gets back up? A disguised Nick Fury (who turns out to be an LMD, as per usual).

Web Comics

• Played with in The Order of the Stick, where two Samurai holding off bad guys for the heroes turn into supporting characters as soon as we learn their names. In fact, one of them had been mortally wounded just as he was about to introduce himself, and as he lay dying, he happened to mention his first name, then immediately realized it was Only a Flesh Wound. He decided to save revealing his last name for an emergency.
  • Also, O-Chul.
• In Errant Story, an inept and cowardly guard from the Veracian Church, named Sandel, suddenly turns out to actually be an undercover spy for the Ensigerum Time-Ninja-Monks! Still only a Mauve Shirt, but that's a hell of a lot better than being an NPC Guard.
• Officer Getskilled from Girly. So far, he hasn't.

Web Original

• The French webnovel Les Aventures de Morgoth has a secondary character named Tiberius Kenny Redshirt, who not only survives but becomes quite a badass.

Western Animation

• In many Looney Tunes and Tex Avery cartoons, a silly character keeps appearing out of nowhere as a Running Gag. At the end, the villain gets fed up and demands the character tells him who he is. He responds, "Don't you know? I'm the hero", then dispatches the villain with a Hyperspace Mallet.
• In Star Wars: Clone Wars Shaak Ti is introduced as a member of the squad of Jedi that Grievous soundly defeats. She survives, and goes on to play a major part in the Battle of Coruscant, protecting Chancellor Palpatine. Not only that, she survives for another 17 years after Order 66, when the vast majority of the Jedi (including all Jedi except Shaak Ti who were present in the Jedi Temple at the time) were killed.
• Used in the Aeon Flux episode "War", each time a Red Herring Shirt kills a Decoy Protagonist and becomes a Decoy Protagonist him/herself.

Other

• By their nature, a lot of Murder Mysteries' culprits.