Dark Mistress

Funny thing. Villains, especially supervillains, are evil, cruel, sadistic bastards. But no matter how many puppies they kick, nuns they throw down mineshafts, or orphans they starve to death, they must have a girlfriend.

This girlfriend is almost always defined exclusively in terms of their relationship to the villain. The role she plays in the plot, however, depends a great deal on how her character is defined. She may be just as evil as he is and actually share his goals. She could be a ditz who doesn't really like evil things but just isn't bright enough to realize she's in a relationship with a very bad person.

Depending on her skill set she might actually be a valued member of the villain's criminal organization. If not, expect much grumbling from his underlings about why the boss even bothers to keep her around.

See Also: Perky Female Minion for a similar character (usually) without an explicit relationship and Unholy Matrimony for a couple that are each villains in their own right. A Lady Macbeth is generally older and goads her partner into committing evil acts. Sometimes subject to a High Heel Face Turn.

Compare The Mistress, Mafia Princess, Daddy's Little Villain.

Examples of Dark Mistress include:

Anime and Manga

C.C. in Code Geass is seen as this in universe. Her role is somewhat muddled by the fact that while she is directly behind Zero's actions, none of the Black Knights actually know what her real role is, and assume she's Zero's mistress. Zero doesn't care, but C.C. at least attempts to correct Tamaki about this, though he doesn't believe her. In reality, she doesn't really fit this trope, having a definite character and ulterior motives beyond helping Lelouch.
Misa Amane from Death Note would be a clearer example. Light definitely isn't a Hero. Then again its debatable whether or not she actually counts as his girlfriend.
One Piece: Depending on your subber or dub, Nico Robin is either explicitly or implied to be Crocodile's bedroom buddy. Considering she still acts similarly, and she seem far from that close with any of the Straw Hat crew, it seems like it's more of an act she learned to put on without actually having any meaning.
Gyokumen Koushu from Saiyuki is the diabolical mistress of the human devouring Gyu-Maoh. Also posing as a main villain through out the plot for attempts to revive him and bring him back to power.
Kanae from Shoujo Kakumei Utena. Heavily deconstructed.
Princess Charlotte from Berserk is head over heels in love with Griffith, who turns into the series Big Bad in an EPIC WAY. Yet she still remains in the dark about all of the horrific actions he has done as well as his true nature. While she's not a big plot device in the story, Charlotte is still of importance to Griffith - not so much as a love interest, but as a Meal Ticket to the throne of Midland.

Comic Books

Harley Quinn from Batman is in an abusive relationship with the Joker. It's a connection she can never quite shake off even when Poison Ivy shows her she can be a supervillain without him, and Batman shows her she doesn't have to be a supervillain at all. In fact, it seems that every time Harley isn't Joker's Dark Mistress, she's Ivy's. In the DCAU it's only after Joker dies that she's finally able to get out of it.
Shriek froms Spider-Man's Rogues Gallery is this to her Axe Crazy lover Carnage, even though she has more depth than the usual type.
- Another Spider-Man example comes from Doc Ock and his lover Stunner.
In Captain America, there was the relationship between Mother Night and the The Red Skull. The Skull being who he is, was horribly abusive to her even though she was completely loyal to him.

Film

Miss Tessmacher and Kitty Kowalski from the Superman movies.
Viper from Doomsday, her death making Sol even more Ax Crazy.
Mirage from The Incredibles. Does a Heel Face Turn.
In G.I. Joe: The Rise of Cobra it's clear Destro wants or even sees The Baroness as this, but she's having none of it.
Vulnavia in the Doctor Phibes movies. Although Phibes (Vincent Price) isn't really in love with her (he's pining for his dead wife), she clearly fills this role.
In Madhouse, film star Paul Toombes (Vincent Price) plays a character named Dr. Death. Paul is annoyed when the producer insists on giving Dr. Death a pretty assistant, and even more annoyed at how unprofessional the actress (who happens to be sleeping with producer) is.
In The Big Heat, The Dragon's girlfriend starts out as the ditzy type, ignoring her boyfriend's vicious tendencies.
Cara Carozza from Dr. Mabuse the Gambler, but she isn't just a mistress for eponymous Dr. Mabuse, but also is his henchwoman, and resident vamp.

Literature

Bellatrix Lestrange from Harry Potter very clearly wishes she was Voldemort's mistress, but Voldemort really doesn't have time for that kind of nonsense. Bellatrix is married, with her husband being mentioned only briefly and the movies cutting him out entirely, but she probably wouldn't mind cheating on him, being evil and all, especially since Word of God is that it's a loveless marriage.
Irma in Bulldog Drummond is probably this, although she doesn't seem to actually do anything except lounge about on the villain's sofa being glamorous.
Left Behind: When Hattie Durham is introduced to Big Bad Carpathia, she's extremely enamored of him. She kind of sours on him when he has her put in a women's prison after she gets pregnant (with his child).
Subverted in Stationery Voyagers as Emperor Alhox isn't really evil, just a Horrible Judge of Character and an Unwitting Pawn. It does frustrate Pinkata a tiny bit that her lover can't deduce for the longest time that he's actually being manipulated by some pretty demonic scumbags.

Live Action TV

In Doctor Who, Lucy Saxon is an unusual example: she and "Harold Saxon" are actually married. But they very nearly make out on national television, so there's that oversexed element, and she's seen sporting a black eye, acting sort of hypnotized, and It Gets Worse.
Heroes featured Danko's mistress, who didn't seem to know but who broke it off with him when she found out what a complete psycho he is.
Harmony in Buffy the Vampire Slayer took a turn as this after becoming a vampire, dating Spike. She was pretty much The Load to his group, and she eventually broke up with him for treating her as an annoying nuisance, but got back together with him several times. Her attempts to go solo as a villain were feeble enough that she eventually got an office job instead.
Gender Flipped on Merlin, in which Morgana was calling the shots, and Agravaine was her thoroughly whipped gofer.

Theatre

Polly Peachum in The Threepenny Opera. Interestingly, she's the main character, and spends much of the play (and the novel) wondering how she got into this situation. It's essentially Marxist commentary on the inability of young women to realize they can live independently of men, and to realize that there's other ways of being financially secure than (a) a criminal life or (b) a life as a sex object. (Later on in the play, Lucy is a straighter example of the trope. She's replaced by Fanny in the novel.)

Video Games

Sarevok in Baldur's Gate has Tamoko, who shows up as the penultimate battle in the game. She can be talked out of it, but canonically she dies during the fight.
A very rare gender-flipped version of the trope is seen in Final Fantasy VIII: Seifer's entire goal in life is to be a sorceress' Knight (protector/henchman/plaything). He ends up being lured in by the villainness almost instantly, being reduced to her lackey for the rest of the plot. Whether or not there's a sexual element to it is left unexplored. Said sorceress is also his foster mother. Or so it seems.
Both Overlord games play this trope very straight, with the evil player character getting the chance to actually choose between several mistresses who pledge themselves to you, ranging from relatively normal women with inexplicable crushes on you, to extremely narcissistic yet beautiful women who enjoy the power and attention.
In Fire Emblem 10, Almedha is revealed as the Dark Mistress to the deceased Big Bad of Fire Emblem 9, Ashnard. Fits perfectly in that they were never married, though really she wasn't "together" with him long, as after their child was born and she lost her powers, he claimed to hold that child hostage to keep her contained, despite the fact that he dumped the child as an orphan long ago. Her son's identity factors heavily into the plot.
- Earlier in the Fire Emblem Series there was Princess Ishtar of the Jugdral series. As the fiancee of the evil Prince Yurius, she fights the protagonists despite a case of My Lover Right or Wrong and partially knowing Yurius is using her.

Web Comics

Alice Evanson in Evil Plan. As the number two to supervillain Dr. Kinesis, their relationship seemed pretty platonic until he literally took her home to meet the parents.
In The Dreamer, Betsy Loring is Gen. Howe's mistress.

Web Original

In the Whateley Universe, Gizmatic's wife, the mother of Jobe Wilkins. Whenever Jobe meets a particularly ditzy girl, he may tell her she reminds him of his mother.

Western Animation

Although prominent in many Batman comics today, it was in Batman: The Animated Series that Harley Quinn was introduced as Joker's girlfriend/henchman in an abusive relationship.
Doctor Girlfriend from The Venture Brothers is a supervillain in her own right, but because she doesn't have any gimmick to theme herself around, she wear a ritzy Jackie Kennedy wardrobe and defines her acts of villainy in relation to her romantic partner, The Monarch. Toward the end of the first season, she dumps him for Phantom Limb, and we see how ineffectual The Monarch is without Doctor Girlfriend around as the voice of reason. Dr. Girlfriend doesn't fare much better either, as Phantom Limb reminds her of why she left him in the first place: Limb is a high-class bore who simply wishes to have her prance around his mansion in skimpy outfits, as opposed to the Monarch, who treasures her intelligence as well as her being "heat incarnate". With the Monarch, she's second-in-command; with Phantom Limb, she serves drinks to house guests.
On Gargoyles, Fox starts out as this to Xanatos, but their relationship quickly evolves into more of an Unholy Matrimony. Their love for each other (and eventually their new baby, Alexander) is a major aspect of their eventual Heel Face Half Turn into an Anti-Villain couple.
Perhaps meant to be parodied on Phineas and Ferb, where Mad Scientist Dr. Doofenshmirtz tries to get a girlfriend, but it never works out, even when he once met a girl as evil as he is. He's also divorced with a child, though his ex-wife apparently never found out what he did for a living.
The Fairly OddParents: Anti-Cosmo has Anti-Wanda. It's a slight variation, as the two are explicitly married.
Averted in Avatar: The Last Airbender. Azula may be a brilliant chessmaster and a highly skilled Manipulative Bitch, but she is too unfamiliar with everyday social skills to form a relationship, so much that the one guy who could have been closer to her was driven away by her loud declaration of We Can Rule Together.
- Toyed with in the relationship between Zuko and Mai though. He's former main antagonist (on the cusp of a Heel Face Turn), and she's the Dark Action Girl who came along on the villainous ride because she was bored and had nothing else to do. In the end though, she is one of the few positive things Zuko has to turn his back on when he leaves the Fire Nation. They even share a cheesy scene where they both agree how they hate just about everything...except each other.
Captain Planet has a Gender Flipped example, sort of—female Mad Scientist Dr. Blight, whose seems to be positively flirting with her computerized Dragon, MAL, in many episodes. (They often call each other "MAL baby" and "Doctor Dearest"...)

Real Life

Entertainment Weekly used "Doctor Girlfriend" as the nickname for the non-fangirl girlfriends and wives of fanboys who are dragged to Comic-Con.
Eva Braun is often portrayed this way.