Love Interest Traitor

This trope is just like it sounds. A particularly insidious type of Romantic False Lead, this character is built up to be the love interest of the (or a) protagonist, but eventually The Reveal 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 Heel Face Turn (often by revealing that she became the mask and genuinely fell in love with the protagonist), they rarely if ever get back together. She is occasionally a Honey Trap, 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 Shocking Swerve.

See also Big Bad Friend. Compare In Love with the Mark.

As a Betrayal Trope, all spoilers will be unmarked.

Examples of Love Interest Traitor include:

Anime and Manga

Occult Academy has an example in Mikaze, who is set up as a love interest for the hero but ends up being the Big Bad.

Film

About half of The World Is Not Enough is spent building up Broken Bird Elektra King as James Bond's new one and only, replacing dear departed Tracy. The film is full of Call Backs 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 Woobie, Destroyer of Worlds who is not only in on Renard the Anarchist's plot to nuke Istanbul, she's co-author of it! Ultimately, Bond is forced to Shoot the Dog in a vain attempt to stop her from telling Renard to move the stolen nuclear submarine into Istanbul's waters.
Something similar to this trope happened in Harry Potter and the Order of the Phoenix, resulting from the adaptation combining the characters Cho Chang and Marietta Edgecombe. In all fairness, it's implied poor Cho was under a truth potion, but that doesn't stop Harry from dumping her for some reason (in the novel, she dumps him over something completely unrelated).
Sky High features a generally Adorkable Nice Guy named Will going out with an older girl named Gwen, whose role is basically showing him the ropes of superhero school in between being sweet on him. Turns out she just wanted the weaponized Fountain of Youth in his basement...
The infamous Garbage Pail Kids Movie 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.
Warriors of Virtue (also known as "that one with the ninja kangaroos") did this, too, with the girl joining up with the hamalicious Big Bad 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 So Bad It's Good-O-Rama that is Spy Kids 3D: Game Over had Juni's girlfriend in the virtual world, Demetra, turn out to be a program inserted into it to trick players into freeing the Big Bad (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 physical exit(!?) so the human characters can escape. She's never mentioned again after this. One can only hope the agency didn't delete the virtual reality with her still in there...
This happened to varying degrees in all three of the Scooby Doo 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:
- 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 centuries-old soul-devouring cat monster...
- Witch's Ghost had Velma's (again, implied to be mutual) crush on horror novelist ~~Stephen King~~ Ben Ravencroft, who turns out to be using the gang in an attempt to free his Eldritch Abomination ancestor (the title character).
- Finally, in 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 Simpsons reference here).
A rare example of this being done with a male character is the Video Brinquedo "spectacular" What's Up? Balloon to the Rescue, in which the lead girl is infatuated with suave Frenchman (read: Devil in Plain Sight Ethnic Scrappy) Jean-Pierre.
Elsa from Indiana Jones and the Last Crusade. 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.
Megara in Disney's Hercules. She is already working for Hades because of having 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 the two end up together.
Double James Bond example: Casino Royale 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 turns out the boyfried is himself a traitor, seducing women with access to state secrets and using them to obtain and sell said secrets. Fortunately(?) she's killed before she finds out about that facet of his personality.

Literature

In the John Gardner James Bond novel For Special Services Bond thinks Nena is the sympathetic Bond Girl, but it turns out she's the Big Bad.
The Sisterhood series by Fern Michaels: 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 Co-Dragons for Big Bad Henry "Hank" Jellicoe - as well as cold-blooded murderers. Ouch!
The big reveal in The Black Saint was that Annie was working for Mustane from the beginning.

Live-Action TV

An episode of Firefly has Mal being married to a young girl for a reward, but she later turns out to be a bounty hunter.
Alias had Michael Vaughn's wife Lauren.
In The Mentalist we have a male example: Grace's fiance in Season 3.

Video Games

Anders in Dragon Age II 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.