You're a Power Trio or Five-Man Band who just got themselves a new Sixth Ranger. Things are looking pretty good right now — your team is expanding, you're making new friends. Looks like things might change permanently!
Wait, changing the status quo? Gasp, heretic!
Surprise! It turns out your fancy new Sixth Ranger is evil! He now knows all of your secrets, how to exploit them, and how to push all of your emotional buttons. You've got a new enemy, and the best part is — you'll have an excuse to keep your team down to a Power Trio or Five-Man Band from now on, so the status quo will remain unchanged.
Just fair warning, this is a trope about betrayal. The names mentioned below may spoil the emotional drama their moment of betrayal brings. However, since Sixth Ranger Traitors tend not to stay with their adoptive teams for long, that may not exactly be much.
Like the Sixth Ranger, The Sixth Ranger Traitor is its own villainous counterpart in a Five-Bad Band, instead reflecting the member of the group who betrays the others, either by pulling a Heel-Face Turn, or simply for their own ends.
The name is derived from Sixth Ranger. Sub-Trope of The Mole.
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Anime & Manga
Kaworu from Neon Genesis Evangelion, although there's enough visual and in-plots hints about people wondering about him that he was deliberately a mysterious character. His character showed up in one episode, which means the time from Introduction to The Reveal about his identity as The Last Angel was about fifteen minutes. Subsequent versions of the story are even quicker. Death & Rebirth reveals Kaworu's betrayal before introducing him! The fact that he's first seen in Rebuild of Evangelion hanging out with Keel Lorentz is a pretty big indication by itself that he's not on NERV's side in the latest iteration of the story, either. note there is subversion in that He's not with SEELE or even the Angels, either in the end. He picks to side with the Lilim, and decides to at least buy some time for Shinji to survive. Not much good that did anyway, but it's the thought that counted.
Corrector Yui - One of the nearest Yui's friends was the true Chosen One to be the titular Corrector, but I.R. chose Yui because an error later explained she was then infected by Grosser its Haruna, and no, its not "a hidden grudge" it was pure Mind Control like it was later demonstrated with Shun before being recruited after Inukai told everyone in the group about it, then after she tried to leave the group (cause Yui was better for the Correctors) Grosser takes her and turned her against Yui, it was fixed on the next episode but she didn't return as a Corrector until the next season. Haruna is eventually cured by Yui herself in an I Know You Are In There Somewhere Fight, goes under temporal retirement (she was still remaining as Mission Control) and, in the second season, she rejoins the team as Sixth Ranger.
In the second season opening of Science Ninja Team Gatchaman, the team gets a new G-2 as a replacement for dead Lancer "Condor Joe" Asakura, much to the others' discomfiture. Surprise! He's a Galactor agent. Double surprise! Joe, Back from the Dead, kills him.
In Death Note, Light Yagami joins the secret police taskforce in charge of finding and stopping Kira, a brilliant but crazy serial killer who's using an Artifact of Doom to anonymously murder anyone he deems evil, which can reach triple digits on a single day. Unfortunately, LightisKira.
It's played with, in that L, the leader of the task force, is never in any doubt that Light is Kira - inviting him to join the task force is just a ploy to attempt to prove his guilt. It works. Eventually. Not that L's around to see it.
Crona from Soul Eater is taken in after turning good, then is manipulated back to Medusa to be The Mole. In the anime, Crona decides to stop working for Medusa and leave the DWMA out of guilt. Maka catches him/her on the way out and convinces Crona to stay and be their ally for the rest of the show. In the manga, Crona disappears just after being discovered, turning out to have been called back by Medusa to her lair. Eventually, Crona kills Medusa, but (as Medusa planned) thought so lowly of him/herself for it that Crona decides to become a Kishin as Medusa planned and cause The End of the World as We Know It. Maka manages to talk Crona back, but by that point Crona had done so much wrong Crona ended up being a Barrier Maiden for the rest of his/her immortal life as repentance.
Meowth of Team Rocket pretends to become a new ally for the heroes of Pokémon Best Wishes for one short Story Arc, but it was just to lure them into a trap in Nimbasa City.
Topher from Runaways. This was less about not changing the status quo, since the team constantly loses and gains members. Topher's betrayal causes the team to be initially suspicious of Sixth Ranger Victor, especially since they know he's the son of a villain (though they don't know which one for a while) and supposedly destined to turn evil. Given that every single fricken one of them is the child of at least one, and in most cases two villains, what do ya expect?
It's also implied to have happened on purpose, because Alex knew all along that Topher was a vampire (or at least evil) and brought him to the Hostel so that the rest of the group could end up fighting him to be toughened up.
In Grant's Morrison's run on X-Men, where the core team starts out as a Five-Man Band, Xorn isn't just a traitor, he's Magneto in disguise! (For a while, anyhow.) Change and (perhaps futile) attempts to escape the status quo were major themes in this run. The first to join the originalFive-Man Band (not counting the Professor) from the sixties, Mimic, also betrayed the team.
New X-Men: Academy X has interesting case with this happening to both New Mutants and their rival team at the same time. Both teams were five groups of friends short of one to make full training squad. Icarus was added to Hellions, whom he barerly knew, because Julian convinced him to join, and Wither joined New Mutants after he returned to school. Once FBI tried to arrest Wither, Hellions decided to bust him out and Icarus, realizing how horrible consequences such act would have, contacted New Mutants and both groups fought. Once Wither was cleared of all charges he realized who his real friends are and asked to be re-assigned to Hellions, while Icarus jumped at opportunity to take his spot on New Mutants.
Nemesis Kid of the Legion in Legion of Super-Heroes. He joined the Legion to spy on them for the evil Khund Empire (and later became a member of the Legion of Super-Villains).
Shun-Day was this to the Legendaries, acting as The Mole to her creator Skroa. She however become the mask and ends up saving the other Legendaries from him. Danael agreed to let her go as a gratitude, but warned her that the next time they'd meet, it'd be as enemies.
In Seventy Two Hours, Bo is this to the wrestlers. When he moves to America, he falls in naturally with the sadistic group, but has no interest in slaughtering innocents with the others; he simply can't escape. During a battle with SABRE, he not only joins the enemy (pulling a complete Heel-Face Turn in the process) but does so in a way that cripples the enemy attack, saving his new friends' lives.
The Dark World arc of the Pony POV Series, Discord's sister Rancor joins him and his minions in fighting the Elements of Harmony's rebellion... only to stab him In the Back (literally) at the opportune moment, in order to steal Destruction's power for herself.
In season 3, Faith is introduced as a second Slayer and hedonistic Shadow Archetype to Buffy. It takes a few episodes, but a fatal accident sets her on a spiral into a Face-Heel Turn. She eventually makes a Heel-Face Turn and goes back to being one of Buffy's most important allies, though.
Rather amusingly, Spike plays the Sixth Ranger Traitor role in season 4 — selling out Buffy and the Scoobies to Adam - despite the fact that he makes no bones about being evil and hating the Scoobies the whole time:
Spike: Tell you what I'll do then. I'll head out, find this girl, tell her exactly where all of you are, and then watch as she kills you.
*shocked silence from Xander and Giles*
Spike: Can't any one of your damn little Scooby club at least try to remember that I hate you all? Just because I can't do the damage myself doesn't stop me from aiming a loose cannon your way. And here I thought the evening'd be dull.
Chester Lake from Law & Order: Special Victims Unit eventually ends up killing a rapist cop who was found not guilty at trial. To Lake's credit, said cop had previously tried repeatedly to kill him, and in the end he made no attempt to conceal his crime. Much more fitting to the trope is The Scrappy Dale Stuckey, who turns out to have masterminded a plot to kill the lawyers and judge in a serial killer's murder trial in order to frame the killer, who he had originally accidentally set free due to screwing up the forensic evidence. He ends up killing the Mauve Shirt CSI tech, and almost ends up killing Stabler.
Leek from Primeval. And Christine. And Philip. If the team (or Nick in series 2, and Abby in series 5) hate somebody, then that person is, guaranteed, to be evil!
Oddly enough, the reality show Dance Moms has an example of this, when the newest mother to the group, Cathy, defects back to her own dance studio, then decides to directly compete against her erstwhile colleagues in an upcoming competition. She even specifically states that now she's seen the Abbey Lee company from the inside, so she can use their own methods against them, going so far as to steal one of the children's music. Like most Sixth Ranger Traitors, she fails spectacularly.
NCIS had both Chip, who inserted himself into NCIS as part of a long term scheme to get revenge on Tony for ruining his career years ago, and Agent Lee, who was an enemy double agent. For a while the team took it for granted that Ziva was one of these, being a double agent for the Israelis, but she eventually proves her loyalty to the team.
Zoey from How I Met Your Mother became one of these, after she grew very close with the gang and even dated Ted. However when Ted said "no" to preserving an old building called the Arcadian, said person revealed to have kept the recording she made of Ted praising the Arcadian despite having promised to get rid of it.
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.
Religion and Mythology
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.
Mordred in the King Arthur mythology
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.
In Final Fantasy IV, when returning to Baron, you run into Cecil's old friend Chancellor Baigan. He "joins" your party (despite the fact that your party is already full (the game doesn't allow six playable characters to be used at once) and he follows along behind you unlike all the other party members who aren't shown on screen during normal gameplay). He is revealed as a monster in disguise about five seconds later.
Hades, Axel, and Xigbar, all from the Kingdom Hearts series. Hades is part of the Disney villain coalition, but eventually abandons Maleficent's plan for universal conquest so he can keep taking potshots at Hercules. Judging by the fate of his co-workers, he had the right idea. Axel and Xigbar are both part of the Organization; the former eventually makes a sort-of Heel-Face Turn, while the latter turns out to be a serious case of Obfuscating Stupidity.
Khunag of Albion joins the party as the sixth ranger. He has little role for awhile. Then at one point, the player must infiltrate the cult he once belonged to, and he's the only guide available. He 'guides' the player into declaring war on the cult, slaughtering their best warriors and eventually killing the leader, with whom he had a score to settle.
In Another Day, the bonus chapter of The World Ends with You, Rhyme, of all people note this is the insanity that is Another Day pulls this on Neku...er, Shuto's team of Tin Pin Slammer-playing heroes.
Harle in Chrono Cross, twice, betraying Lynx for you because you are Lynx and then betraying you for the Dragons after you beat FATE.
Aydana Kozlova in Ghost Recon: Shadow Wars is a subversion of this trope. She seemed to betray the team and released the enemy that The Squad had just captured. It turns out that she only released him so the team would be able to track him down and lead them to the Big Bad. Then she joined back your team and was Easily Forgiven.
Dark Pit from Kid Icarus: Uprising was created by Pandora to be an Evil Knockoff of Pit who'll fight for the Underworld Army. 10 seconds after his creation, he defects and becomes a neutral Wild Card during most of the game until he joins with Pit near the end of the game.
Rusty Pete may also count, since he only allys himself with Ratchet to find the body of Angstrom Darkwater so that he can revive Captain Slag.
Terra from Teen Titans. In the original comics, she was The Mole working for Deathstroke, but in the cartoon, she underwent a Face-Heel Turn. If it wasn't obvious enough that Terra as a Titan wasn't going to last long, the episode following her joining the team has her suspiciously absent save a very brief non-speaking cameo in the end, and the title of the next episode when she actually appears as a member of the team? "Betrayal". And then her next two appearances in which she actually invokes this trope? "Aftershock" (Which is a two part episode).
William from Code Lyoko. Although he is not evil, he got cocky to the point where XANA succeeded in taking him over. The surprise factor was actually legitimate, because the build-up was so long.
Super Robot Monkey Team Hyper Force Go provides somewhat of a twist on this with Mandarin, because he was originally introduced as "The Sixth Monkey," but it's explained in the beginning of the episode that he was actually the leader of the Hyperforce before Chiro and they had to get rid of him because he turned evil.
Foxglove from Chip 'n Dale Rescue Rangers helped the Rangers mainly so she could steal the episode's Plot Coupons they found, and give them to the villain/goons she worked for... and also because of an affection for Dale, which lead to her Heel-Face Turn back to the Rangers.
Also, Desiree D'Allure, Monterey Jack's old crush, does this as well... except unlike Foxglove, she stays evil, and gets away with it! (Well, not completely.)
On Titan Maximum, Gibs used to be the team's Smart Guy and The Lancer. The series starts with his reappearance after disappearing for a few years and announcing that he's the new Big Bad in town.
In Robotix, the Protectons eventually revive another of their frozen comrades into a robot body, but unknown to them, Nemesis switched things so the body actually houses the mind of one of his minions, Venturak.