This trope is just like it sounds. A particularly insidious type of RomanticFalseLead, this character is built up to be the love interest of the (or a) protagonist, but eventually TheReveal comes that she (and it's usually she) is with the bad guys.

This character, as mentioned, is almost always female, and while she may pull a HeelFaceTurn (often by revealing that she [[BecomingTheMask became the mask]] and genuinely fell in love with the protagonist), they rarely if ever get back together. She is occasionally a HoneyTrap as well as TheMole, but not always; sometimes the fact that she's a love interest and the fact that she's a traitor have nothing to do with each other. Done badly, this can be a ShockingSwerve.

See also BigBadFriend. Compare InLoveWithTheMark.

'''As a [[BetrayalTropes Betrayal Trope]], many if not all spoilers will be unmarked.'''

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!Examples:

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[[folder:Anime & Manga]]
* ''Anime/SeikimatsuOccultGakuin'' has an example in Mikaze, who is set up as a love interest for the hero but ends up being the BigBad.
* [[WhiteHairBlackHeart Kaworu]] [[EnigmaticMinion Nagisa]] in the original ''NeonGenesisEvangelion'' anime. Arrives to NERV HQ as a new pilot who immediately takes an interest in [[HurtingHero poor, depressed Shinji]] [[HopeSpot when everyone else had either died or become estranged from him]], turns out to be [[ApocalypseMaiden the final angel.]] Since he genuinely cares for Shinji despite all of this, Kaworu ultimately goes with the HeelFaceTurn option and performs a HeroicSacrifice to save both Shinji and humanity via allowing Shinji to kill him. Notable for being [[HoYay a same-sex example.]]
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[[folder:Comic Books]]
* ''ComicBook/{{Runaways}}'': Alex is set up as Nico's love interest, but is ultimately revealed as the team's mole.
* As mentioned below, in the "Judas Contract" arc of TeenTitans Terra is set as this to Beast Boy. Noticeable in that this original Terra was far, ''far'' nastier than the cartoon one: while cartoon!Terra was a sort-of DarkMagicalGirl, comics!Terra was a ManipulativeBitch.
* Jezebel Jet dated Bruce Wayne while trying to break him on behalf of the Black Glove in ''[[ComicBook/GrantMorrisonsBatman Batman RIP]]''.
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[[folder:Films -- Animated]]
* This happened to varying degrees in all three of the animated ''Franchise/ScoobyDoo'' movies that are generally considered "the good ones;" in fact, it may be part of the reason they're regarded so highly that they give the gang other characters to play off of:
** ''[[ScoobyDooOnZombieIsland Zombie Island]]'', the first, played it straight, with Fred developing feelings for a {{Moe}} Cajun chef named Lena, with hints dropped that she may like him, too. Nope, turns out she's a [[ReallySevenHundredYearsOld centuries-old]] [[NinjaPirateZombieRobot soul-devouring cat monster]]...
** ''[[WesternAnimation/ScoobyDooAndTheWitchsGhost Witch's Ghost]]'' had Velma's (again, implied to be mutual) crush on horror novelist Ben Ravencroft, who turns out to be using the gang in an attempt to free his EldritchAbomination ancestor (the title character).
** Finally, in ''[[WesternAnimation/ScoobyDooAndTheAlienInvaders Alien Invaders]]'', Shaggy's girlfriend Crystal isn't a traitor, exactly, but she is connected to the film's haunting; she's an alien. A real one. She and Shaggy part on amicable enough terms, but she ultimately has to go back to her home planet (insert ''[[TheSimpsons Simpsons]]'' reference here).
* Megara in Disney's ''Disney/{{Hercules}}''. She is already working for Hades because of having [[DealWithTheDevil made a deal with him]], so when Hades discovers that Hercules is still around, he uses her as a pawn to get at him. Also a rare example in which [[InLoveWithTheMark she]] ''[[InLoveWithTheMark does]]'' [[InLoveWithTheMark fall in love with him]], the two end up together.
* A rare example of this being done with a ''male'' character is the VideoBrinquedo "spectacular" ''What's Up? Balloon to the Rescue'', in which the lead girl is infatuated with suave Frenchman (read: DevilInPlainSight EthnicScrappy) Jean-Pierre.
* [[BigBad Prince]] [[VillainWithGoodPublicity Hans]] from {{Disney/Frozen}} is a RareMaleExample. He pretends to fall in LoveAtFirstSight with [[ThePollyanna Princess Anna]], planning to marry her and then arrange an [[MakeItLookLikeAnAccident "accident"]] for her older sister [[TheHighQueen Queen Elsa]], and thereby become the King of Arendelle.
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[[folder:Films -- Live-Action]]
* ''Film/JamesBond''
** About half of ''Film/TheWorldIsNotEnough'' is spent building up BrokenBird Elektra King as Bond's new one and only, replacing dear departed [[Film/OnHerMajestysSecretService Tracy]]. The film is full of {{Call Back}}s to Tracy, comparisons between the two, and includes the most explicit Tracy reference of the Brosnan era, all the while implying that Elektra may not be completely right in the head (she enjoys fiddling with ice cubes mid-coitus, for one thing). ''Then'', at around the midpoint of the movie, it comes out that Elektra is in fact a WoobieDestroyerOfWorlds who is not only in on [[BigBad Renard the Anarchist's]] plot to nuke Istanbul, she's [[BigBadDuumvirate co-author of it]]! Ultimately, Bond is forced to ShootTheDog in a vain attempt to stop her from telling Renard to move the stolen nuclear submarine into Istanbul's waters.
** ''Film/CasinoRoyale'' has Vesper, playing along with Bond until she steals the money to get her boyfriend (who she's still in love with) back. However, it's suggested that she did fall for Bond as well, cutting a deal to save his life in exchange for going through with the theft. Bond's RoaringRampageOfRevenge in the next film culminates with the reveal that the original boyfriend is himself a traitor, seducing women with access to state secrets and using them to obtain and sell said secrets. Fortunately(?), Vesper's killed before she finds out about that facet of his personality. She dies thinking that she has at least saved the lives of the two men she loved.
* Something similar to this trope happened in [[Film/HarryPotter the film version of]] ''Literature/HarryPotterAndTheOrderOfThePhoenix'', resulting from the adaptation combining the characters Cho Chang and Marietta Edgecombe. In the movie it's implied poor Cho was under a TruthSerum, but that doesn't stop Harry from dumping her (in the novel, ''she'' dumps ''him'' over realising that they simply didn't work out, since [[spoiler: Cho was still ''massively'' hurting over the death of her boyfriend Cedric, murdered by Voldemort in front of Harry]]).
* ''Film/SkyHigh'' features a generally {{Adorkable}} NiceGuy named Will going out with an older girl named Gwen, whose role is basically [[MentorShip showing him the ropes of superhero school in between being sweet on him]]. Turns out she just wanted the weaponized FountainOfYouth in his basement...
* The infamous ''Film/TheGarbagePailKidsMovie'' has the equally-infamous Tangerine (infamous because she looks about ten years older than her early-teens leading guy; the actors are a year apart in real life). Her loyalty turns out to be to a street gang, and the poor girl gets pooped and vomited on at the end of the movie by the title characters for it.
* ''Film/WarriorsOfVirtue'' (also known as "that one with the ninja kangaroos") did this, too, with the girl joining up with the [[HamAndCheese hamalicious]] BigBad to avenge her brother, who had been accidentally killed by a kangaroo ninja. She's ultimately killed by her boss when she has a change of heart and helps the hero escape.
* The SoBadItsGood-O-Rama that is ''Film/SpyKids 3D: Game Over'' had [[KidHero Juni's]] girlfriend in the virtual world, [[LittleMissBadass Demetra]], turn out to be a program inserted into it to trick players into freeing the BigBad (who's also trapped there). In all fairness, she didn't ''want'' to do it, and ends up rebelling against her programming and holding open the virtual reality's [[FlatWhat physical exit(!?)]] so the human characters can escape. She's never mentioned again after this. [[FridgeHorror One can only hope the agency didn't delete the virtual reality with her still in there]]...
* Elsa from ''Film/IndianaJonesAndTheLastCrusade''. To be fair, she genuinely loved Indy (and...uh...Indy's ''dad'' too, apparently), but she's single-mindedly obsessed with getting her hands on the Holy Grail to the point of working with Nazis.
* In the 1966 ''Film/BatmanTheMovie'', Bruce falls in love with Kitka only to find out she's Catwoman (and definitely a villain in this incarnation).
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[[folder:Literature]]
* ''Literature/JamesBond''
** In ''Literature/ForSpecialServices'', Bond thinks Nena is the sympathetic BondGirl, but it turns [[spoiler:out she's the BigBad]].
** Subverted in ''Literature/{{Icebreaker}}'' with [[spoiler:Bond's Finnish girlfriend Paula Vacker]], who initially seems to be on [[ThoseWackyNazis NSAA]]'s payroll, but turns out to be an agent of SUPO (Finnish intelligence agency) working within the organization. The trope is played straight with the other love interest [[spoiler:Rivke Ingber]], who is revealed to be working for them when the former's allegiance is made clear.
** Two women accompany Bond throughout ''Literature/NobodyLivesForEver'' as he makes way to SPECTRE's base, and once he reaches it, one of them is revealed to be working for them.
* ''Literature/SisterhoodSeries'' by Creator/FernMichaels: Starting with ''Final Justice'', Little Fish is set up to be a love interest for Countess Anne "Annie" Ryland de Silva and Stu Franklin is set up as a love interest for Isabelle Flanders. However, by ''Cross Roads'', both relationships are falling apart. The final blow comes when Little Fish and Stu Franklin are revealed to be CoDragons for BigBad Henry "Hank" Jellicoe - as well as cold-blooded murderers. Ouch!
* In the [[Literature/XWingSeries Wraith Squadron trilogy]] this is the situation from the perspective of Myn Donos in that he falls in love with Lara Nostil only to find that she was the infiltrator that destroyed his squadron. Though more generally it is more of a DatingCatwoman scenario in that the reader fully knows who she is. It is also more complicated in that while she was originally an infiltrator, [[BecomingTheMask she defected to the New Republic without telling them.]]
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[[folder:Live-Action TV]]
* An episode of ''Series/{{Firefly}}'' has Mal being (unintentionally) married to a young woman for a reward. But she turns out to be a con artist, intent on killing the crew (by leaving them to die) and selling their ship. Her appearance in a later episode reveals this to be her standard operating procedure: seduce, steal, leave 'em for dead, repeat...
* ''Series/{{Alias}}'' had Michael Vaughn's wife Lauren.
* In ''Series/TheMentalist'' we have a male example: Grace's fiance in Season 3.
* In ''Series/{{Supernatural}}'', Sam's love interest Ruby turns out to have been manipulating Sam into freeing Lucifer.
* ''Series/{{Teen Wolf}}'' has this happen to Derek Hale, twice!
* In the episode "Turn, Turn, Turn," ''Series/{{AgentsofSHIELD}}'' reveals that [[spoiler: Ward was one for Skye]].
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[[folder:Videogames]]
* While he's not a full LoveInterest due to his plot being mostly DummiedOut, Bishop of ''NeverwinterNights2'' for a female protagonist. He will always betray her to the King of Shadows at the end of the game, and there is nothing she can do to stop it; at best she can persuade him to walk away from the battlefield, but he won't rejoin her side. [[spoiler:[[RocksFallEveryoneDies And then a bunch of rocks fall on them both]].]]
* Anders in ''Videogame/DragonAgeII'' if he is your love interest. No matter what you do, he will blow up the Chantry near the end of the game in order to start a war between the two major factions, even if you are trying to be neutral.
* [[spoiler: Penelope]] of the ''Franchise/SlyCooper'' games. [[spoiler: She betrays Bentley]] due to his devotion to his friends over her, and becomes a major antagonist in [[VideoGame/SlyCooperThievesInTime the fourth game]].
* Played with in FireEmblemAwakening: [[spoiler: The Avatar ''can'' be one of Prince Chrom's love interests if female, and has massive HoYay with Chrom if male... but the part where s/he kills Chrom and thus kicks off the BadFuture happens whether the Avatar is a male or a female ''and'' whether a girl Avatar is married to Chrom or not.]] Also, [[spoiler: s/he doesn't do it ''willingly'' but only when his/her body is taken over by the EvilGod Grima, whom s/he is supposed to be the vessel of... and the plot of the game as a whole is about ''preventing'' the BadFuture (and thus the Avatar's betrayal) from taking place.]]
* The BigBad of ''VideoGame/NineHoursNinePersonsNineDoors'' turns out to be [[spoiler:none other than June/Akane]]. Most of her behaviour is acted, but the affection appears to be genuine.
-->'''WordOfGod:''' "My intention was to create the world's worst heroine."
[[/folder]]

[[folder:Western Animation]]
* ''WesternAnimation/TeenTitans'': In addition to being a SixthRangerTraitor, the series's version of Terra was this to Beast Boy. It was implied that (unlike the original Terra), she ''did'' like Beast Boy back, trying to save him from the rest of the Titans' fate, but not enough to choose him over Slade. [[spoiler: She ultimately rebelled against Slade when Beast Boy confronted her upon her deeds, and then performed a HeroicSacrifice to save the whole city as well as the Titans themselves. ]]
** This trope was revisited in the next season when Raven falls for a handsome magician trapped in a book--who turns out to actually be an evil dragon that tried to use her as the key to return to the world, and whom Raven must re-seal and defeat. Notably, Beast Boy is the one who consoles her afterwards.
* In the ''SupermanTheAnimatedSeries'' episode "[[Recap/SupermanTheAnimatedSeriesS3E8SupermansPal Superman's Pal]]", Jimmy discovers that his new girlfriend, Tina, is working with Metallo and only wants him as bait for Superman. The last we see of her is him locking her in a closet before going off to help the superhero.
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