Fan Works / Sixth Ranger Traitor

Cinders and Ashes: the Chronicles of Kamen Rider Dante introduces Shimashima, a producer who once worked with Setsuna and is now working with the main heroes to stop her final creation, Altair, from destroying the multiverse. However, midway through the story, it is revealed that Shimashima was merely a cover for his true identity: Yuusuke Shimazaki, Setsuna's father, and that his true allegiance is with Altair out of revenge for her death.
Code Lyoko: EG: During the fifth arc, this role is filled by all three Dazzlings.
A Hollow in Equestria: During the Changeling Arc, the Mane Six become convinced that Ulquiorra has turned against them. Turns out that he was actually following Celestia's orders the whole time.
In the Hunting Series, Agent Moore betrayed the System, which led to Neo almost being kidnapped by agents of the Marovingian when he was just a child.
My Shinobi: In the Hanzo Raid arc, Naruto himself turns out to be a Deep Cover Agent for the Hebijo team, which results in the Hanzo team getting curb-stomped... and severely demoralized by the unexpected betrayal.
The Supermen: Wesker starts Playing Both Sides under various aliases, joining the Justice League, the 1000, and the League of Assassins, with the intent of betraying them all as it suits his purposes.
Miyako in the Death Note and Phantom Thief Jeanne crossover To Kill a Thief is unknowingly helping Light. When she realized of what he wanted, she turns her back on him.

All Assorted Animorphs AUs: "What if they had a chance to do it all over again?" gives the main Animorphs the chance to prevent David from becoming one, thanks to being sent back to the start of their adventures.

In Tinted Green, Asami pretends to be on Korra's side, acting as one of her moles amongst the Equalists.

In 72 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.

Dark Studios Kids Next Door: The aptly named "TRAITOR" centers around the Teen Ninjas searching for one in their ranks, emphasizing that the organization is a Flock of Wolves — the whole group has been compromised by the KND, to the point where it's effectively one big game of Whack-A-Mole.

In Devil's Advocate, Takato joins the Digidestined to help them defeat the Digimon Emperor. However, as soon as the Emperor dies, Takato plans to turn on the others and kill them.

Mobile Suit Gundam Alpha has four Gundams: Alpha, Betta, Gamma and Delta. Only three of them are on board the Ark Royal; Delta was captured and taken over by the Titans.

The Power of Seven: The Death Eaters attempt to set Pansy Parkinson up as one of these, believing that Harry will readily welcome her into his group simply because she's attractive. However, they swiftly figure out the scheme, but welcome her into the group so that they can Feed the Mole.

A Diplomatic Visit: The epilogue of the fourth story, The Diplomat's Life, reveals that Lady Aputsiaq is secretly working for the same being who was served by Subtle Dancer from the Doa-verse, and stole the Gem of Sacanas from where Twilight had it stored and turned it over to them. Twilight herself remains unaware of the thief's identity.
During 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 The Retelling of Pokémon Colosseum, Miror B. is secretly working against Cipher from within their ranks, recruiting Skrub to their cause.

In Personality Conflicts, this is Tommy's greatest fear. When Trey gets arrested, he immediately figures out that there's one of these hidden in his line of succession, trying to get him out of the way.

Sonic: Evil Reborn Zero has Etero, who also happens to be the first competent human to join the otherwise all-animal cast.

The Legend of Spyro: A New Dawn: Cyros joins the group with the sole purpose of tricking them, stringing them along with Half-Truths.

Daisy's Adventures: During Elemental Odyssey, Grekka briefly joins the team, betrays them, then gets a Redemption Equals Death moment.

Black Crayons: While being held hostage, Annabelle overhears Starscream and Barricade discussing matters in Cybertonian. While she doesn't know much of the language, she gleans enough to realize that they're talking about a traitor among the Autobots' ranks, and warns the Autobots about it. Said traitor turns out to be Sentinel Prime.

Sixth Ranger Traitor / Fan Works