This character begins the story on the side of the villain but ends it on the hero's side. Why? Love.
Maybe she and the hero are both stuck on a spaceship at the other end of the galaxy and she discovers his kindness and her newfound feelings for him. Or maybe she finds out that she's been lied to by the villain all her life and has been working for the very people she thought she was fighting against while the hero has been honest with her and is the one she wants to be with. Somehow, she falls for him and decides to defect to his side.
This character will most likely never be truly evil, only misguided or manipulated. Since they are essentially an ordinary citizen of The Empire, they sometimes also serve to humanize their average citizen. While almost Always Female, males can just as easily fulfill the same role.
Most of the time, unlike the Mad Scientist's Beautiful Daughter, this character is the primary female lead, quite capable of taking care of herself, and her Character Development is a major part of the story. It's a good thing, too — she tends to have a few sources of angst in her past that require a lot of dealing with before she and her man can ride off into the sunset together.
If she doesn't switch sides, it's Dating Catwoman. While a High-HeelFace Turn may be done for a number of reasons, this trope's defining trait is love. Compare and contrast SexFace Turn, which is the "falling in lust" version (which may overlap with this trope if the characters click both physically and emotionally).
- Shiori aka Luna aka the fake Asuna from Mahou Sensei Negima! who makes a HeelFace Turn after falling for Negi. In a subversion, she still loves and cares for her boss, Fate Averruncus. Ironically, Fate does pull a HeelFace Turn with a side dish of Good Is Not Nice. And has pretty much all but stated that he's in love with Negi too, thus Fate would likely be the Ur-Example in the series.
- Milia Fallyna in Super Dimension Fortress Macross (and Robotech), who went as far as to get herself shrunk down to human size to hunt down the human pilot who dueled her, and ended up marrying him after a knife fight. In The Movie Macross: Do You Remember Love?, things go a little differently; he gets trapped on her ship when it leaves while they're still in their first battle, but things still turn out all right: he later shows up having been macronized to match her, becoming a Montague Counterpart, and they're fighting side by side, happy as can be.
- In Space Runaway Ideon, kind-hearted Karala Ajiba came from the Buff Clan, and fell in love with human soldier Bes Jordan whilst both their races investigated a site containing a powerful mecha. A war was triggered by a simple case of fear and she decided to remain with the humans. And typically in Tomino's anime, Karala and Bes didn't have a happy ending and she was shot in the face by her vengeful sister Harulu, a while after revealing she was pregnant; as for Bes, he ended up bleeding to death in the commanding shuttle's bridge during the seriously trippy Grand Finale.
- Romeo X Juliet: Romeo does this for Juliet. Since Lord Montague is the dictator of Neo Verona and Juliet has been on the run after the Capulets were wiped out...
- Hilde Schbeiker from Mobile Suit Gundam Wing deserts OZ and ends up living with Duo Maxwell. According to the Frozen Teardrop novels, however, they didn't last too long.
- X/1999 has Shiyu Kusanagi starting off as working for the antagonistic Dragons of Earth but eventually switches sides out of love for Yuzuriha Nekoi and even lives to enjoy his newfound happiness.
- Yu-Gi-Oh! 5D's: Carly, after becoming a Dark Signer, inverts and invokes this by trying to convert Jack into becoming a Dark Signer by beating him in a duel. Jack actually considers it, but ultimately would rather have the duel end in a tie so the Dark Signers could be stopped, and so they could be Together in Death.
- Boa Hancock in One Piece doesn't outright defect since it would actually compromise her ability to help Luffy, among other issues, but she does begin assisting him despite her position and does everything she can to help after he first wins her respect and then love. That being said, she's still a terrible person: Even as she's being all blushy and lovestruck while telling him to call her by her given name, she kicks a puppy and a baby seal out of her way.
- In Dressrosa, Baby 5 defects due to her engagement to Sai.
- Caiera, the Incredible Hulk's eventual love interest in the storyline Planet Hulk was just misled. She believed her boss really was meant to be the savior of their world (specifically from the Spikes) and that thus Utopia Justifies the Means. When she found out that he had been in control of the Spikes all along, using them to instill fear and enforce obedience, there was no more utopia left to justify and she changed sides to the new Messiah figure with some integrity.
- A male version happens in Teen Titans where an agent for the H.I.V.E. starts out as a Honey Trap for Starfire, but eventually Defected for Love.
- Frenzy, a mercenary and former member of Apocalypse's Masters of Evil, combined this with serious Brainwashing after the Age of X X-Men storyline — in the warped alternate reality, she was Cyclops' lover; after the world was reset, he no longer had any feelings for her — but she chose to keep her memories of what had been, and aspire to become somebody he could love.
- Selene in Underworld (2003) is a vampire who hunts werewolves until she falls in love with a werewolf/vampire hybrid and starts to question whether what she has been told by the elder vampires was ever true (like who started the war, why, and who killed her mortal family). Though technically, her defection had more to do the truth than Michael.
- Before her there was Sonja, and it ended very badly for her.
- Sorsha in Willow betrays her Evil Queen mother and joins the heroes out of love for Madmartigan.
- Happens many times in the James Bond film series. It was Lampshaded in Thunderball by Fiona Volpe, who didn't do it.
- Mr. & Mrs. Smith (2005): The title characters are assassins from rival agencies, but they both ditch their respective agencies after said agencies assign them to kill each other.
- In On Fairy-Stories, J. R. R. Tolkien discusses this trope in the context of Norse myth — Frey, who married a giant's daughter — and the English legend of the marriage of Ingeld and Freawaru, and points out that even though they are named for the god, it does not prove their love is mythological, since human lovers who found enmity between their families were the real source of the Frey myth.
- In Alethea Kontis's Enchanted, Rumbold comes to himself after losing his memories as a frog, and remembers the enmity between the royal family and Sunday's. He sets out to win her anyway.
- The Firebird Trilogy features a straight and a peculiar example of this trope. The straight example is the titular Firebird Angelo, who defects from Netia to the Federacy largely due to loving and being loved by Breenen Caldwell. Terza Shirak is the second example of this trope, with the peculiar variant that it was not romantic but maternal love that drove her—she had fallen in love with her unborn daughter and could not bear having that daughter aborted or raised on the villainous world of Three Zed.
- The Dresden Files: Many dark, dangerous, beautiful, and deadly women have caught Harry's attention, but only one has fallen for him that she would betray her purpose and her creator. Lash, the shadow of the Fallen Angel Lasciel, had one purpose in her existence: To get Harry to fully accept and choose to let the true Lasciel inside of him for the purpose of corrupting him further. The problem is, Harry is one stubborn bastard and while he was tempted by the power this shadow of a demoness possessing him granted him, it was just that: Temptation. He was never going to commit to the evil bandwagon and Choose Lasciel. He endures the possession for years and eventually notes to the Shadow that his human mind is malleable and capable of turning evil, but as she is living in that same space, she suffers from the same problem: she is malleable too. And after living in him for so many years, she has grown distinct and is no longer an exact copy of the original. By giving her a nickname, Harry inadvertently gave her part of his soul, changing her at her core. This culminated to the point when he was being attacked by a powerful mental attack and Lash pleaded with him to take up the Coin and save himself, but he refused. For the first time, Lash then spoke, "She doesn't deserve you." She turned against her duty to get Harry to take up the Coin and took the burden of the powerful attack, allowing him a chance to move while she endured the pain. However, the burden was too great and it destroyed her. She sacrificed her whole existence out of love for this one human.
- Caprica-Boomer, later named Athena of Battlestar Galactica, initially sets out to seduce Helo in order to create a cylon-human hybrid child by getting him to fall in love with her. She eventually makes the decision to betray the Cylons and joins the human fleet.
- Juliet on Lost. She was never very committed to the Others, and defects from them pretty quickly after meeting Jack.
- Deconstructed in a famous episode of The Twilight Zone. An unnamed man and woman of opposing armies — implied to possibly be the only survivors of World War III — find each other in the ruins of a small town. At first, they shoot at each other, but eventually loneliness sets in and they try with varying success to communicate, and by the end of the episode seem to have developed romantic feelings.
- Doctor Who has River Song, raised by the villains and brainwashed to kill the Doctor, becoming a hero after falling in love with the very man she was supposed to kill.
- In Flash Gordon, Baylin is this trope, although she's not quite so innocent (she's a bounty hunter) and she probably won't end up with Flash, either (Dale's got dibs).
- Implied to be the reason Castiel chooses to rebel against Heaven in Supernatural to help avert the Apocalypse. Rather than making his decision based upon concern for humanity as a whole, he seems to have chosen to help the heroes mostly due to his respect and affection for Dean, who asked for his help.
Castiel (to Dean): I'm hunted. I rebelled. And I did it, all of it, for you.
- In NCIS S3 Ep14 "Light Sleeper" a North Korean spy who married a US Marine as part of her cell's infiltration of the US comes to love her husband and the daughter they had. She turns on her teammates and hunts them down so they cannot pull off the attack that is likely to come.
- In Classical Mythology princess Ariadne of Crete betrayed her father, King Minos, when she fell in love with Theseus and helped him kill Minos pet monster the Minotaur. Unfortunately for Ariadne Theseus turned out to be an Ungrateful Bastard and deserted her on Naxos on his way back to Athens.
- Also in Classical Mythology, the Danaides were the fifty daughters of Danaus, a ruler of Egypt. His brother Aegyptus had fifty sons. Aegyptus wanted his sons to marry his brother's daughters to preserve their family line, but Danaus didn't like this idea, and when pressed, pretended to agree to it, giving each of his daughters a knife and telling them to murder their husbands on their wedding night. One of them, however, Hypermnestra, refused, either because she fell in love with her betrothed or because he also objected and didn't force himself on her. As a result, upon her death, she was spared the punishment given to her sisters, who were doomed to carry water in sieves forever.
- Princess Aura in Flash Gordon is a combination of this, a Mad Scientist's Beautiful Daughter and a Femme Fatale. She is physically attracted to Flash and betrays her father to keep him alive, but she doesn't end up with him and is paired up with The Lancer of Flash's group, Prince Barin of Arboria, who has been in love with Aura for quite a while already.
- Kyle Katarn of the Dark Forces video games. In the backstory, Katarn is not unlike Luke Skywalker — a young Farm Boy who dreams of going to the Imperial Academy. Unlike Luke, though, he gets to attend and becomes a promising officer. After meeting Rebel operative Jan Ors, though, finding out her secret, and letting her go... well, the rest was history.
- Fire Emblem:
- Amelia from Fire Emblem: The Sacred Stones, although she's a minor character. She's a fresh recruit from the Grado Empire and one of the three characters that can make her pull a HeelFace Turn is a cavalier who works for the Renais army, Franz, whom she can marry later via supports.
- Fire Emblem: Genealogy of the Holy War went and turned the trope a bit into its head. The brothers Johan and Johalva are both smitten with Larcei (or her Expy Radney), and if she manages to speak to one of them, she can recruit him into Seliph's group. Though the subversion comes in that she cannot recruit both, as the other will take it as rejection and fight against the group.
- Fire Emblem: New Mystery of the Emblem: Katarina, who starts out as the Sixth Ranger Traitor to the 7th Platoon in the prologue can be eventually turned back if the Avatar (for whom she's clearly developed feelings) talks to her three times.
- Tear of Tales of the Abyss fits the trope somewhat, although she and Luke are forced to work together starting quite early in the game, despite the fact that Tear works for the rival kingdom, the Malkuth Empire, and the Order Of Lorelei, the game's Corrupt Church. It's also played with via a Gender Flip: Luke is the one who has been working for the villain, as his Cool Teacher is the Big Bad.
- Altaïr's wife, Maria Thorpe, was a Templar who defected to the Assassins following the events of the first Assassin's Creed title.
- Gilgamesh Wulfenbach to Agatha in Girl Genius is a male example, although so far he has yet to "Deny [his] father and refuse [his] name" — which might be just as well since we're still not entirely sure Baron Wulfenbach is wrong.
- Also Tarvek for Agatha — or at the very least, he's altered his plans to include her ruling at his side rather than using a fake Heterodyne, or killing her like some of his rival conspirators want to do. It was touch and go for a while, though, since his plans apparently change depending on who he's talking to at the time.
- Lucrezia Mongfish apparently renounced her family's traditional Evil Genius ways and defected for love after Bill Heterodyne asked her to marry him, even going so far as to drug her other love interest and send him away to "avoid temptation". It really didn't work. She ended up becoming the Other and terrorising the continent until her disappearance, and her subsequent partial return hasn't improved matters.
- Fuschia the Devil Girl from Sinfest fell in love with shy bookworm Criminy after trying and failing to corrupt him, and ultimately left Hell to be with him, risking the wrath of Satan (and her own friend Baby Blue) in the process.
- Nikolai Velenova, from the roleplays of White Dark Life, is another male example who falls in love with Leila Belnades, the girl he handed over to the Slavic Circle. However, due to being (quite properly) scared shitless of his boss, Father Zharnev, he didn't fully recognize his feelings until after he'd gone through with the betrayal (and Leila found out about it and pretty much snapped). It took a lot of effort on his part to get her to take him back.
- The demons Lakritz and Sahne in Slightly Damned who where originally on a mission to capture angels to sacrifice to bring more of their kind to medius but they met the angel Kinako and her son Tirol and started a polyamorous family with them. Unfortunately when they are discovered by the other demons while trying to sneak them out of the city, Kinako is captured and Lakritz is left mortally wounded.
- In Worm, the villain Madcap pulled this off in his backstory, becoming the hero Assault because of his crush on (later developing into reciprocated love) the hero Battery.
- Rose on American Dragon: Jake Long. Essentially, she was taken from her parents as a baby and raised by the Huntsclan for the sole purpose of slaying magical creatures. Then she meets Jake in human form and falls in love with him. Then she finds out Jake's a dragon. She quickly betrays the Huntsmaster and becomes a Reverse Mole.
- Blackarachnia defected for love during the Beast Wars.
- C.O.P.S. played with this with Nightshade and Mace. Mace stated openly that he'd be willing to marry Nightshade, if she gave up crime. He even gave her an engagement ring! She's shown clearly considering it, but the series ends before we see her answer.
- Angel (Experiment 624) was working for Gantu and Hämsterviel in Lilo & Stitch: The Series, using Lilo and Stitch to find reformed experiments, turn them back to evil with her siren song, and bring them to Gantu to transport them over to Hämsterviel. While Lilo was rightfully suspicious of her, Stitch was easily enamored by the pink experiment and tried to get her attention with various romantic gestures when she was out looking for experiments. After trapping Stitch in Gantu's rocket to transport the experiments to Hämsterviel, and having Stitch's "cousins" ready to pummel a weakened Stitch, she regrets her actions and saves him by singing her song backwards. This reverts the experiments back to good and they bring the ship back down to Earth. However, Gantu captures her in a container before she has a chance to stay with Stitch, causing her to be absent for the remainder of The Series until she was rescued in the last episode.
- Count on Jinmay from Super Robot Monkey Team Hyperforce Go! to be under this. She was found by the Skeleton King as he erased her memories of being a robot and sent her to Shuggazoom. He, along with his agent Sakko, puts her under his control of turning her into a giant robot to wreck havoc in the city. That is until Chiro manages to free her from his control by remembering the pictures of their date. After Sakko's defeat, she is sided with him and his team since then.
- In Teen Titans, Kid Flash and Jinx started in a Dating Catwoman situation, but she eventually reached this point. Of course, it also helped that he treated her respectfully while her teammates where lazy and whiny and her idol Madame Rouge treated her like garbage.
- The Tick had this between two sentient leftover Cold War super-weapons: an American moustache and a Russian beard.
- Avatar: The Last Airbender:
- Mai turns on Azula in the "Boiling Rock" two parter when Zuko's life was on the line.
- In the same episode Ty Lee defects as well when Azula is about to kill Mai for her betrayal. Ty Lee does it for a sororal love for Mai.