The Turncoat is the guy who switches sides at some point to help out the other side. Can be a hero who turns bad, or a bad guy who suddenly decides to help the good guys, but usually it's just anyone who thinks that the switch will benefit them personally.
If they're discovered to have been bad the whole time, they're The Mole. If they're a bad guy who's been secretly good, they're the Reverse Mole. If they're faking their switch, they're a Fake Defector or Heel Face Mole. If they switch back and forth, they're going through the Heel Face Revolving Door. The Defector From Decadence often falls under this.
It's interesting to note the origins of the phrase "turn coat". Back in the day (we're talking horse and musket age) soldiers defecting to the other side would turn their coats around so the liner would face the outside, to indicate they were not a soldier of the enemy. Of course, one only did this when you were far enough away from your own lines...
Not to be confused with Turn Coat.
- Beyblade: Kai Hiwatari ofswitches sides six times. First, he's part of the antagonistic Blade Sharks. Then he joins the Bladebreakers (first switch). Then he switches sides and joins the Demolition Boys (a Face Heel Turn, and switch #2). Then he switches sides again and rejoins the Bladebreakers (three). He stays with them until he goes over to the Blitzkrieg Boys (the Demolition Boys renamed, and switch number four). His next move is to join the bad guys of season three, BEGA (five). Of course, he realizes the error of his ways and switches back again (six).
- Bleach: Uryuu Ishida goes from being an enemy of Ichigo and the shinigami to an ally... albeit with loopholes helping him along the way to protect his pride.
- Flarejet in Transformers Operation Combination, a Decepticon who used to be an Autobot and a friend of Firefoad
- Justin Law in Soul Eater. Appears to be working for the antagonists (of which there are a few groups), specifically Noah, with a view to uncovering the Kishin. Now, technically, this was what Shinigami asked him to do (drive Asura out of hiding by making his presence known). But his methods might not go down too well. If this is a ploy, he's really getting into it.
- The Legend of Zelda: Four Swords: Vio (Fake Defector) and Shadow (Heel Face Turn).
- Inuyasha: Kagura desired her freedom from her enslavement to Naraku. As the story develops, she helps Sesshoumaru against Naraku more and more and begins to aid Inuyasha's group against him, too. When she takes active measures to save Kohaku's life, Naraku finally ends hers.
- A Song of Ice and Fire is filled with these. Particularly sellswords, who will abandon battle and even turn against those who hired them and join the enemy if the tide is against them.
- Theon Greyjoy is dubbed Theon Turncloak after betraying House Stark.
- Harry Potter: Peter Pettigrew, who sold out two of his best friends, Lily and James Potter, to Voldemort, and then framed a third friend, Sirius Black, for the whole thing. He considered turning back (just a little bit!) after Harry Potter saved his life, but the mere thought of betraying Voldemort caused his magical prosthetic arm to strangle him.
- Esmer in The Last Chronicles of Thomas Covenant is chronically unable to choose a side, to the annoyance of everyone involved. He has the rare distinction of being able to switch sides several times a day and the personal power to pull it off.
- Warhammer 40,000: In Ben Counter's Horus Heresy novel Galaxy In Flames, Lucius motivated by envy of how Tarvitz took command, betrays the last survivors of Horus's treacherous attack.
- In Edgar Rice Burroughs's Chessmen of Mars, the ancient I-Gos is perpetually praising his days. So thorough is his admiration that he changes his loyalties on realizing who is The Hero.
Then I did not fully realize the cowardice of my jeddak, or the bravery of you and the girl. I am an old man from another age and I love courage. At first I resented the girl's attack upon me, but later I came to see the bravery of it and it won my admiration, as have all her acts. She feared not O-tar, she feared not me, she feared not all the warriors of Manator. And you! Blood of a million sires! how you fight! I am sorry that I exposed you at The Fields of Jetan. I am sorry that I dragged the girl Tara back to O-Tar. I would make amends. I would be your friend. Here is my sword at your feet.
- In Discworld, Nobby Nobbs is famous in times of war for exactly this. He hovers around the edge of the battlefield, swiping boots off the fallen soldiers and will just move in with whoever he thinks is winning. The generals used his uniform as an indicator to tell who was winning.
- In Mercedes Lackey's The Black Griffon, two of Urtho's generals seem to be carrying the Idiot Ball for most of the book; they're constantly losing troops of all species by attempting flashy, dangerous tactics which would lead to glorious victories, if only they actually worked; it later comes to light that these losses are intentional, as they have been working for the enemy for some time.
- Conan the Barbarian:
- In Rogues in the House, Conan's Backstory includes being betrayed by a woman. After he's out of prison, he tracks her down and (since she is a woman) abducts her to drop her in a cesspit.
- In The Hour of the Dragon, Publio turns on Conan because Conan's presence might reveal that his wealth springs from dealing with Conan when he was a corsair.
- In "The Scarlet Citadel" Amalrus, who lured Conan into an inverted Cavalry Betrayal.
- Happens in John Le Carre, along with moles, with regularity, since that's the whole point of having agents and double agents. As one character put it:
Don't feel too bad. Christ himself only had twelve, and one of them was turned.
- Buffy the Vampire Slayer: Faith (Face-Heel, then Heel-Face) and Spike (Heel-Face, then Face-Heel, then back Heel-Face). Not to mention Angel (Heel-Face when he got a soul, Face-Heel when he lost it, etc. ad nauseum). Also Andrew (Heel-Face) and Anya (Heel-Face, brief Face-Heel, then back to Heel-Face).
- Skinner in The X-Files.
- In Lost, Juliet turned good while Michael turned bad and then good again.
- In the final episode of Star Trek: Deep Space Nine, the Cardassian ships turn on their Dominion allies and aid the Federation Alliance fleet. Though the fact that one of their major cities on their homeworld had been wiped from the face of the planet might have had something to do with it. Gul Dukat fits this trope alone, when he realises that the military is going to be overthrown he switches to back the civilians ("Way of the Warrior")
- Final Fantasy IV: Kain Highwind switches sides four times throughout the course of the game. He was under More Than Mind Control from about one hour into the game to about one hour until the end of the game. Golbez just liked to screw with the heroes by slackening his control on Kain.
- Final Fantasy VII: Reeve Tuesti, controlling Cait Sith is The Mole for the bad guys at first, but quickly swaps sides wholeheartedly once he's found out, even taking a risk for the party through sacrificing one of his puppets for their cause.
- Tales of Symphonia: Yuan switches from allying with the heroes to trying to kill them so often even the other characters comment on how hard it is to keep track. It makes sense, as he's a double agent within the villains' organization, and leads his own organization opposing the villains that is easily confused with said villains. So he never really switches sides, just motives. Pretty much half of the cast betrays Lloyd at one point or another. Which makes you wonder just how naive he is.
- Zevran from Dragon Age may join your party after you foil his attempt to assassinate you for Loghain. If he does, depending on his Relationship Values, he might later turn around and rejoin the enemy.
- In Avatar: The Last Airbender, Azula joins forces with the imprisoned Long Feng and takes control of his Dai Li agents to capture the Avatar, overthrow the king, and free Long Feng from prison. Once the Earth Kingdom is under their control, Long Feng wants to dump her and rule by himself. However, during the time of her leadership, the Dai Li have come to greatly appreciate the ability and ruthlessness of Azula, so they just ignore his order to capture her, waiting for them to prove who is more worthy of their loyalty.