Capulet Counterpart

The Capulet Counterpart begins the story on the side of the villain but ends it on the hero's side. Maybe she and the hero are both stuck on a spaceship at the other end of the galaxy and need to work together to get home. 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. Or maybe she just gets fired by the villain and her new job involves working with the hero.

Whatever the reason my be, she starts working with the hero, probably reluctantly at first. But when a situation comes where they can actually sit down and talk -- and one won't be long -- she and the hero will actually find they have much in common. There's no fighting the power of love or friendship. Soon she's one of his inseparable True Companions, and quite possibly his soulmate, too.

The Capulet Counterpart is almost Always Female, and like her predecessor has a remarkably high chance of being the hero's love interest. Because the Capulet Counterpart is essentially an ordinary citizen of The Empire, she sometimes also serves to humanize their average citizen.

Most of the time, unlike the Mad Scientists Beautiful Daughter, the Capulet Counterpart 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 -- the Capulet Counterpart 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.

This character type is becoming a very common RPG female lead.

If she doesn't switch sides, it's Dating Catwoman.

Anime and Manga

 * Myria Parino (that's my spelling and I'm sticking to it) 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, 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 sight 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,
 * Ironically, Romeo's the Capulet Counterpart and/or Mad Dictator's Handsome Son for Juliet in the Revision anime Romeo X Juliet. Since Lord Montague is the dictator of Neo Verona and Juliet has been on the run after the Capulets were wiped out...
 * Aina Sahalin in the Gundam OVA The 08th MS Team ends up in this role, though in the end she doesn't so much join the Feds with Shiro as much as they both quit being soldiers entirely. Which is probably for the best; Shiro is suspected as being a spy because of the relationship, and Aina probably isn't too popular with Zeon bigwigs after the stunt her older brother pulled.
 * An argument could be made for Shiro being Aina's Capulet Counterpart because he fulfills the "doesn't know the Big Bad personally" part better than she does, as he is a grunt while she is part of a relatively high up family as well as the main villain's sister. Also, Shiro is the one who gets chewed out by his superiors (and branded a traitor by some) for the entire thing while only one of the two characters on Aina's side aware of it is harsh (her brother, predictably enough). Ace Pilot, Badass Normal, and A Father to His Men Norris Packard (who is something of a father figure to Aina, complete with fighting Shiro to see if he's worthy) is much more sympathetic to Aina and Shiro's plight than most of the other characters, so in a way Shiro also fulfills the "berated by his fellows" part better than her.
 * Also he's the one with the dark childhood incident. Well, not childhood per say, but "dark past incident".
 * Also about as good as their could expect, considering that, this being Gundam, most Capulet Counterparts and/or their would-be significant others usually end up dead. sometimes at the hands of the other (whether they know it or not).
 * Hilde Schbeiker from Gundam Wing definitely fits this trope. She deserts OZ and ends up living with Duo Maxwell, earning her the everlasting hatred of the yaoi fangirls.
 * in Gundam 00.
 * San in Princess Mononoke fills part of this requirement, as she and Ashitake start out on opposite sides but finally converge. She just won't stick around
 * Apollonius in the backstory of Genesis of Aquarion.
 * Not sure if Athrun Zala from Gundam Seed would count. He's the 2nd main male character but he and Cagalli were in a similar situation. They were both from mostly opposing sides, they met as enemies, Athrun took her a prisoner, which led to them getting to know one another and found that they like each other.
 * Athrun is also from the "empire" that is opposing the protagonist for most of the series (not that the Earth Federation is exactly "good") and is pretty ignorant of what the top brass are really doing. In the end, he does oppose what they are doing and ultimately "betrays" and fights against them.
 * Stella in the sequel Gundam Seed Destiny definitely counts. Of course, she never switches side, but she wouldn't fight against her love if she had the slightest control.
 * All of Magical Girl Lyrical Nanoha is basically about this: there is a bunch of misguided (predominantly female) souls, who are not truly evil, just not knowing any other purposes in life, who get into a fight with Team Nanoha. Nanoha, being, well, Nanoha, proceeds to dish out massive ordnance friendship, and the lost souls suddenly discover that Team Nanoha has cookies, free hugs for everyone, and secure jobs at TSAB. And by the next season, all of them join Team Nanoha themselves.
 * X 1999 pretty much pulls a Gender Flip of this between Yuzuriha Nekoi and Shiyu Kusanagi, in that he starts off as working for the antagonistic Dragons of Earth but eventually switches sides out of love for Yuzuriha and even lives to enjoy his newfound happiness.
 * All of Magical Girl Lyrical Nanoha is basically about this: there is a bunch of misguided (predominantly female) souls, who are not truly evil, just not knowing any other purposes in life, who get into a fight with Team Nanoha. Nanoha, being, well, Nanoha, proceeds to dish out massive ordnance friendship, and the lost souls suddenly discover that Team Nanoha has cookies, free hugs for everyone, and secure jobs at TSAB. And by the next season, all of them join Team Nanoha themselves.
 * X 1999 pretty much pulls a Gender Flip of this between Yuzuriha Nekoi and Shiyu Kusanagi, in that he starts off as working for the antagonistic Dragons of Earth but eventually switches sides out of love for Yuzuriha and even lives to enjoy his newfound happiness.

Comic Books

 * Caiera, the Incredible Hulk's eventual love interest in the storyline Planet Hulk. Unlike most examples, she was well aware of the fact that her boss was a piece of work, and her working for him was not entirely willing.
 * She wasn't unwilling, just misled. She believed he really was meant to be the savior of their world (specifically from the Spykes) and that thus Utopia Justifies the Means. When she found out that he had been in control of the Spykes all along, using them 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.
 * Julie-Su is this to Knuckles in Archie's Sonic the Hedgehog comics.

Film

 * Averted in Indiana Jones and The Last Crusade. Elsa, an otherwise obvious Capulet Counterpart not only fails to end up with the hero, but suffers a (deserved) Karmic Death.
 * Elsa's death seemed to have been more of a case of Too Dumb to Live, as she avoided any major Shoot the Dog moments in the movie, and only died because she tried to take the Grail after being explicitly told that doing so would be very bad.
 * In Metropolis, Freder is the son of the semi-evil Capitalist overlord, and Maria is a young Socialist reformer. It turns out okay, though.
 * Jay, the childhood friend of Shua in Sky Blue.
 * Many Bond Girls are like this, particularly Pussy Galore in Goldfinger.
 * Yelena in xXx mostly fits this trope, though her backstory is a bit more complicated.
 * Selene in Underworld is a vampire who hunts werewolves until she falls in love with 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).
 * Freya in Outlander
 * Dana in Camp Rock 2. She is the daughter of the rival camp's owner
 * Sorsha in Willow. She is the daughter of evil Queen/Sorceress Bavmorda.

Literature

 * Lale of The Assassins of Tamurin.
 * Mara Jade of the Star Wars Expanded Universe was this to Luke. She still hated Luke when she first met him, not to mention it took nearly ten years to realize that she's in love with him.
 * Fateofthe Jedi: OK, we don't know if Vestara will be this, but at least Ben tries to turn her into.
 * Let's take a look at what Vestara does in Vortex, shall we?
 * This is the whole point of Son of the Mob by Gordon Korman. (Her dad's an FBI agent, his dad's the head of the mafia.)

Live Action TV

 * Aeryn Sun in Farscape. She doesn't so much switch sides as get kicked out of her side for being in too close proximity to escaped prisoners and an unknown species of alien.
 * Actually she gets the boot for questioning Big Bad Crais' version of events (that John Crichton deliberately destroyed the Prowler flown by Crais' brother). The No Fraternisation with Aliens rule was just an excuse.
 * Caprica-Boomer, later named Athena of Battlestar Galactica Reimagined 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 Three--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.
 * Chakotay and especially B'Elanna Torres from Star Trek Voyager fit this trope. They were part of the Maquis until they got stuck on a Federation space ship and joined the crew.
 * Cara from Legend of the Seeker is ordered to capture Richard but he persuades her to help him defeat Darken Rahl. They don't fall in love though.
 * Deconstructed and later reconstructed with with Faith on Angel: She starts out as a good guy on Buffy the Vampire Slayer then joins the Big Bad and finally teams up with the heroes again when Angel shows her The Power of Friendship/Optimistic-Existentialism.

Newspaper Comics

 * Princess Aura in Flash Gordon is a combination of this, a Mad Scientists 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. In the 2007 series, Baylin is a much closer version of 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).

Video Games

 * Elly from Xenogears is a classic Capulet Counterpart.
 * Vivian from Paper Mario the Thousand Year Door is a non-romantic Capulet Counterpart in the U.S. version, where she's female.
 * Sheena from Tales of Symphonia. In a variation, it's Colette, her rival Love Interest for The Hero, that makes her start to rethink her opposition to the party.
 * Celes of Final Fantasy VI is in some ways a spiritual predecessor of Elly.
 * Celes had already rebelled against the Empire (and was imprisoned for it) when she joins the plot, and her eventual falling for Locke does not affect her standing with the Empire.
 * Beatrix from Final Fantasy IX.
 * Anise Azeat from Galaxy Angel II. However, while the player can end up with her in the games, outside canon goes with Apricot as the winning girl.
 * A rare Montague Counterpart is 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, 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.
 * 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 Heel Face Turn is a cavalier who works for the Renais army, Franz, whom she can marry later via supports.
 * This trope fits Shelke, from the Final Fantasy VII spin-off Dirge of Cerberus, like her mako-infused spandex suit. Dead parents, kidnapped and trained to be a lethal member of the Tsviets, reluctantly joins the hero, lolicon-ish hints with protagonist...yup.
 * Trishka in Bullet Storm.
 * 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. It's also played with.
 * Elena of Yggdra Union. She manages to not become the lead male character's love interest, though--possibly because the game's playable character is female and Elena's Heel Face Turn comes about halfway through it.
 * Amanda in Jack Keane
 * Karin of Shadow Hearts: Covenant.
 * World of Warcraft's Jaina Proudmoore. She doesn't actually defect, as such, but she's probably the most sympathetic Alliance commander toward the Horde for at least two expansions, and her relationship with Horde Warchief Thrall is (probably at most, despite the hopes and wishes of the vast majority of shippers) one of friendship.

Webcomics

 * Gilgamesh Wulfenbach to Agatha in the webcomic Girl Genius is a male example (Montague Counterpart?), 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.

Western Animation

 * The Tick had this between two sentient leftover Cold War super-weapons:
 * 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.
 * COPS played with this with Nightshade and Longarm. Longarm stated openly that he'd be willing to marry Nightshade, if she gave up crime. She's shown clearly considering it, but the series ends before we see her answer.
 * Zuko from Avatar: The Last Airbender is one of the few male examples in fiction.
 * Both Dinobot and Blackarachnia became capulet counterparts during the Beast Wars. Even though Dinobot was brought back from the dead as a bad guy the two were never on the same team at the same time. At least, not long enough to hold a conversation.