This trope takes place when a Five-Man Band or Power Trio finds a new ally. Things are good for a while, however, at a certain point in the story, it turns out said new member was The Mole all along, working for the villains or, worse, is the Big Bad themself. He now knows all of the party's secrets, how to exploit them, and how to push all of their emotional buttons. They've got a new enemy, but, on the plus side, they'll have an excuse to keep their team down to a Power Trio or Five-Man Band from now on, so the status quo will remain unchanged.
Like the Sixth Ranger, the Sixth Ranger Traitor is its own villainous counterpart, instead reflecting the member of the group who betrays the others, either by pulling a Heel–Face Turn, faking one, simply for their own ends, or on rare occasions against their will.
In Role Playing Games and other videogame genres, they tend to be the Guest-Star Party Member. If the party has to fight them at some point in the story, then they went from Temporary Party Member To Villain.
As this is a Betrayal Trope, unmarked spoilers abound. Beware.
- Dog Soldiers - Megan, who was a werewolf in secret from the start and believed the squad could somehow save her, betrays the soldiers after coming to the conclusion that either they weren't there to help her and that they'd kill her if they found out her secret, or that their chances of surviving the night were nil, or a combination of both.
- In The Faculty, Marybeth is this in a meta sense. While the other five protagonists didn't know each other before the events of the movie, they are pretty obviously based on the group from The Breakfast Club (Stan's counterpart is Andy, Delilah's is Claire, Casey's is Brian, Zeke's is Bender, and Stokely's is Allison), leaving one to wonder where Marybeth fits in. The answer: she doesn't. She's the queen alien leading the Puppeteer Parasites that are taking over the school.
- In Grave Encounters 2 Lance appears out of nowhere, having apparently survived the first movie and shows up to aid the remaining survivors. Too bad for them he is taking orders from the hospital itself which speaks by scratching words into the walls or following him around invisible in the darkness of the hospital. To make this even worse, he kills Trevor and attempts to escape the hospital himself. too bad the demons of the hospital double crossed him too.
- In Mission: Impossible – Fallout, Walker is set up to be the IMF's new additional member and a Foil to Ethan. Turns out he's actually the terrorist John Lark who's been acting as The Mole within the team to frame Ethan and steal the plutonium for Solomon Lane.
- In Seven (1979), Mailei reveals herself to The Mole inside The Syndicate: providing to information to Harris and the feds on their plans to take over Hawaii, and then sent to help the Seven to stop them. However, she later turns out to be a double agent, providing The Syndicate with information about how much the feds know, and tries to kill Cowboy.
- X-Men: First Class: The Mutants are initially happy simply to have found others like them, but the group eventually splits over fundamental differences in ideology.
- After a fun evening spent demonstrating their powers, Angel abandons the team after their CIA safehouse is attacked, agreeing with Shaw that Mutants should not have to fear the humans.
- After the combined US and Cuban militaries attempt to kill the Mutant squad that has just kept the two countries from initiating what promised to be a costly and destructive war, Magneto and Raven decide it's dangerously naive to continue to work with the CIA-backed team. Magneto will go on to use the leadership skills he learned from Charles and the increased abilities Charles helped him hone to establish the Brotherhood of Mutants.
- Animorphs: David. Note that no one really wanted him to join, and he didn't either, but he'd found the morphing cube and his parents had been taken by Yeerks, while the Animorphs barely rescued him from being taken as well. While he doesn't side with the Yeerks, his goal being becoming rich with an army of morph-capable minions, he runs the team ragged by demonstrating how difficult it is to track and fight someone with their powers. He ends up permanently trapped in rat morph, and his fate (a Mercy Kill by Rachel or left to live as a rat) is left deliberately vague.
- In The Clockwork Prince, Jessamine Lovelace betrays the Institute because of her love for Nate.
- Senna from Everworld is sort of an odd example—she technically put together the group, but keeps flitting in and out of it herself (until book #7, when she stays with them until her final betrayal). She's definitely a Token Evil Teammate, though, and after a while doesn't even try to conceal the fact that she's manipulating them, though she keeps her exact plans hidden. Unfortunately, the others have to put up with her, since she's MacGuffin Super-Person and the only one who can bring them home.
- In Harry Potter: Severus Snape is actually a Sixth Ranger Traitor for the villains, earning himself a (sudden) special place in the hearts of many readers who had previously hated his guts.
- Villains by Necessity: Robin, the last and sixth member of the party to join, is a spy from Mizzamir. However, he isn't really evil, and switches sides after learning their quest is right.
- Hushai plays the role of The Mole for King David in the story of Absalom's rebellion. The guy even has the balls to lampshade it in front of Absalom.
- CM Punk played the good version of this in WWE's ECW revival, infiltrating the New Breed in order to break them apart from within.
- EVIL played the... well, evil version of this in NJPW, betraying Los Ingobernables de Japon and joining Bullet Club in 2020. LIJ had been a full-blown heel stable when he joined, but had transitioned to being tweeners by the time he left.
- MJF played the evil version twice in AEW:
- In late 2019, he allied himself with Cody Rhodes and his Nightmare Family, seemingly turning face, but betrayed Cody a few weeks later, cementing himself as a heel.
- A year later, he joined AEW's top heel stable, the Inner Circle, but manipulated them for months, driving original member Sammy Guevara from the stable and attempting to usurp Chris Jericho as leader. Jericho then booted MJF from the stable in March 2021, only to find that MJF started his own heel stable. MJF's group, soon unveiled as The Pinnacle, beat the Inner Circle down, effectively turning the latter face.
- The Crossoverlord: The Green Avenger was this. Justified, because she was brainwashed by Smiling Man.
- Eikre from RPG World was heavily implied to be one of these when he joined the party. Subverted when it turns out he was on the level and the real mole was a flower that he bought from a random NPC to give to Cherry.
- Emily from Our Little Adventure turned out to be a Puppeteer Parasite Manchurian Agent of the Souballo Empire.
- Critical Role: Campaign Three: In terms of guest player characters:
- Dusk, an elf paladin who joined the group after getting rescued and telling them she was close with Fearne's long-lost parents, was later revealed to be the Unseelie changeling Yu, who was hunting Fearne's parents down over their theft of the Moontide Crown and used Fearne to lure them out of hiding.
- After Bells Hells was split up across Exandria in the wake of the Apogee Solstice, Orym, Ashton, and Laudna ended up in Issylra with three strangers, Deni$e, Prism, and Bor'Dor. After telling everyone he was a simple shepherd with no magic experience and fighting alongside them, Bor'Dor was pressured by Deni$e into confessing his lies, immediately attempting to kill the group and revealing that he was one of the Ruby Vanguard who was fighting against Bells Hells on the Apogee Solstice. He became the first player character in the history of Critical Role to be murdered by another party member because of this. Even worse, according to his player, Bor'Dor's original plan was to attempt to kill them mid-teleport.
- In MarsQuest, Sjin joins up with Lewis Brindley, Simon Lane and Duncan Jones to get to Mars and rescue the captive Sips. Once the goal is achieved, he tries backstabbing Team J.A.F.F.A, but this fails horribly.