In 2012, during one of the famous Superbowl commercials, M&M maker Mars introduced Ms. Brown, an icy businesswoman voiced by Vanessa Williams, to go with the other M&M "spokescandies". Brown was the last regular color to be characterized, and since Green was the only other female out of the set, it seems Mars wanted to kill two birds with one stone.
Anime and Manga
In Digimon Adventure, Sora and Mimi were the only two girls on a seven-person team, until the introduction of Hikari, cute little sister of the male lead. Digimon Tamers had a slew of Sixth Rangers, going from a cast of three with one girl to a cast of ten with near-equality.
Rain herself becomes a secondary Affirmative Action Girl later, when Allenby is Brainwashed and Crazy and she takes up the task of Defusing The Tykebomb. She already had hints of it the beginning, though, being able to withstand the "painful" suiting up process and handling the Shining Gundam in Shinjuku despite lack of specific training.
Hilde Schbeiker from Mobile Suit Gundam Wing. Though she's a rather decent pilot and had military background prior to befriending Duo, she was mostly there to balance the ratio for a while and become the local Wrench Wench...
Textbook example in BB Senshi Sangokuden: the second named female character Sonshoukou Gerbera is the resident tomboy, and the first, Chousen Qubeley, is The Chick complete with healthy relationship to Ryofu Tallgeese, and gets Stuffed In The Fridge for it.
While Dragon Ball always had a few female characters scattered throughout the supporting cast, the anime-only Dragon Ball GT was the only time a girl regularly joined in on the world-saving: Pan, Goku's granddaughter.
The inclusion of the Golden Age Wonder Woman in the Justice Society of America could count like that. The team was founded just by male heroes.
The 2012 JSA reboot makes Hawkgirl into a founding member. Bonus points since she's a Latina as well.
During Super School's first run in The Beano there were originally four super kids three male one female and then Bananagirl was added to the cast balancing the sexes quite a bit.
Sanabi, in the live-action Death Notemovies. Similarly, the role of the Third Kira was given to an adapted version of (attractive twenty-something female) Kiyomi Takada, originally merely a Kira-supporter and a go-between for Light and Teru Mikami (the fourth Kira), when character-wise, an adapted version of (unattractive older male) Kyosuke Higuchi would have worked just as well, although in Takada's defense, she had a role in the live-action story.
Most of the cast of the first Toy Story was, well, a collection of boy's toys. The only female character, Bo Peep, was fairly minor and only existed because the boy shared a room with his baby sister (and so Woody could have a Love Interest). In Toy Story 2, Mrs. Potato Head was also fairly minor, but Jessie was one of the mains. Toy Story 3 gives Mrs. Potato Head a larger role, and also adds Barbie, though Bo Peep was actually cut entirely.
In Barbie's case, it was also the fact that the movies had permission to use the character for the first time. Originally, Bo Peep (the aforementioned only female character in the first movie) was supposed to be a Barbie, but Mattel refused. It wasn't until they saw how the movies caused sales of Mr. Potato Head toys to spike that they agreed.
The film adaptation of Voyage of the Dawn Treader adds a second girl to the crew - Gael, a girl who stows away after her mother is kidnapped. This would have created a Plot Hole for when the Dufflepuds kidnap Lucy as in the book, they do so because only a girl can read the spell to free them. However the film remedies it by having one of the Dufflepuds note that Gael is also a girl but that Lucy has a book next to her, indicating she knows how to read.
Tia Dalma, a minor character in the second Pirates of the Caribbean movie, joins the crew in At World's End so that Elizabeth isn't the only woman on the ship (since Anamaria vanished in Dead Man's Chest).
Although she did become quite important in the next movie...
In the adaption of SWAT, a newcharacter was added, Chris Sanchez, a woman who has repeatedly applied to join SWAT but was rejected for being a girl, with her number of police brutality accusations (Actually just perps who were embaressed at beingtaken down by a girl) given as a reason for not allowing her to join. and Then came Samuel L. Jackson...
Harry Potter and the Order of the Phoenix adds Tonks to the team of good badass adults we're used to as well as Ginny Weasley and Luna Lovegood to the main group. Although Ginny was in the story before this book, she was just a background character who had no real impact on storylines. Here she becomes a proper Action Girl and joins Harry, Ron, Hermione and Neville in the fight in the Ministry. Luna as well then makes the main group have three girls and three boys.
On the evil side we originally had Bellatrix as the The Dark Chick. Alecto Carrow then appears as another female Death Eater, but this trope fits Bellatrix's sister Narcissa better; she technically appeared earlier and she's only the wife of a villain, but her role gets greatly expanded in the final two books, culminating in a Heel Face Turn that allows Harry to kill Voldemort.
Ginny technically does this twice. She becomes important in the second book but is reduced to background character for the next two before officially becoming prominent permanently from the fifth book onwards.
Phenomena: Even though Millian first appears in the 3rd book does she have an unique personality and is a good addition to the series and has many fans.
The second book of A Song of Ice and Fire, A Clash of Kings, introduces many female warriors into the already large cast of characters, including Asha Greyjoy, Brienne of Tarth, Ygritte, Meera Reed, as well as fire priestess Melisandre. Martin explores how each fits into the male-dominated world of Westeros. The fourth book, A Feast for Crows, also introduces the Sand Snakes, a group of sisters who were brought up to be badasses by their father.
Speaking of original Japanese versions, Super Sentai started the practice with Choudenshi Bioman, but from then it swings back and forth between playing the trope straight and averting it (of the 28 seasons since then, 13 ran on The Smurfette Principle). Engine Sentai Go Onger played with this by starting off only with Saki Rouyama (Go-on Yellow), then introducing Miu Sutou (Go-on Silver), the first ever female Ranger with Sixth Ranger status.
One could make a case for Aisha Campbell as well (Trini's direct replacement after she was Put on a Bus); even though the Yellow Ranger, by now, was a bit more "girly", the corresponding Sentai still has her in the place of a guy (Kazu/Kirin Ranger from Gosei Sentai Dairanger).
Not exactly balancing out the gender balance, Shuki from Kamen Rider Hibiki was a lot more competent and stayed on the show longer than Larc and Femme. And by longer, we mean two episodes as opposed to just thirty minutes.
Inverted on The Sarah Jane Adventures. In the original pilot, the gender ratio was three females (Sarah Jane, Kelsey, and Maria) to one male (Luke). When the series began, Kelsey had been replaced with Clyde to balance out the gender ratio (and because Kelsey's actress was rather difficult.)
The Hawaii Five-0 reboot/remake changed Kono into a chick, because, well, they needed a chick.
Gender Flipped in Warehouse 13 the original cast was Mrs.Fredrickson Mika, Leena, Artie, Pete, with Claudia joining after a few episodes in making it 4 girls and 2 guys. Season 2 introduced female H.G. Wells then dropped that character at the end of the season and has introduced Agent Jinks in Season 3 making 4 girls and 3 guys. 5 girls and 3 guys if you count H.G. Wells who is Commuting On A Bus
Gender Flipped in Charmed as the ratio was typically the three sisters and one male character. Andy filled this role in season 1 but then season 2 replaced Andy with Dan and gave Daryl and Leo more prominent roles. Cole got added in season 3, making the main cast an even split of 3 men and 3 women.
Another Gender Flip in H 2 O Just Add Water with a lead cast of three girls and one guy. Season 2 adds Zane to the opening credits and then season 3 adds Will (though dropping Louis as well).
In the actual Samurai Seven anime series every single warrior in it is male. Thus, certain fan fics for it actually did include female fighters.
Also, there turns out to be more to Miang than what the player is told at first. She is not only manipulating Ramsus from behind the scenes, but has been doing the same to the whole world for 10,000 years—she is one of the two BigBads in the story and for all intents and purposes, the physical avatar of their world's god.
Typically, the only female recurring character in the Super Mario Bros. series is Peach. In the spin offs, they literally throw in as many female characters as they can find. While many of the additional characters in the Mario Spin Offs (Daisy, Baby Peach, and possibly Rosalina) could be accused of being Affirmative Action Girls, only Toadette and Baby Daisy were created specifically for this purpose.
The original Metal Slug had two characters, the guys Marco and Tarma. Metal Slug 2 revealed their female counterparts, Fio and Eri. Fio and Eri have stuck around ever since... except for when Tarma and Eri were put on a different mission in Metal Slug 4 and replaced with Trevor and Nadia.
The original version of Final Fantasy III had a party of four generic boys. The remake changes the blue Onion Knight to a girl.
Surreally, Dissidia makes its onion knight male, anyway, worried that having a whopping five of its twenty-four characters female would be far too many. Its sequel works the other direction, introducing eight new characters, four of them female.
In Warcraft III, female units would usually be relegated to supporting magical roles (sorceresses for Humans/High Elves and Banshees for Undead), up until Night Elves had females filling all the primary attack unit roles.
Also, the expansion pack for Warcraft II added Alleria Windrunner, an elven ranger hero and the only female unit, to the story.
In the original Nintendo 64 version of Diddy Kong Racing, the only female character was Pipsy the mouse. In order to even the balance a bit (and fill the empty spots left by characters Nintendo no longer had permission to use, along with making the game less of an In Name Only addition to the Donkey Kong Country franchise), Diddy's girlfriend Dixie Kong and her younger sister Tiny Kong were added to the Nintendo DS version.
In Final Fantasy IX Freya and Eiko (and Quina depending on your views) fill these roles. Your base party had just Garnet as the only girl and she was pretty useless at that point, only being useful as The Medic. Freya is the first fifth party member you get (as a main character rather than just guest party members) and is the first other female main character to be introduced in the story. Eiko then joins the group halfway through the second disk where Garnet is again the only girl on the team. Beatrix also joins the group as a guest party member for a fight early on disk 3.
Aqua in Kingdom Hearts is the first properly playable female character, not just a bonus addition. And she is awesome.
Viola of Stationery Voyagers. It was strongly implied that Neone would join shortly after the first launch, so she doesn't count. But Viola appears from seemingly out of nowhere in the second season, and quickly convinces the team to let her join, in spite her eccentricities being confusing to them.
Failed attempt: The creators of Homestar Runner tried on a few occasions to develop other female characters for the site to supplement Marzipan; none ever saw the light of day, except in a bonus short on the DVD appropriately titled "Why Come Only One Girl?" They eventually gave up, deciding that the sub-feature Teen Girl Squad would be their new female outlet.
In-continuity, Bethany of The Escapist's Game Dogs is literally this-in spite of a behavioral record as long as any arm you'd care to present, stuffed full of reprimands for violence that would get anyone else fired twenty-seven times over, she keeps her job explicitly because of affirmative action rules mandating a certain number of female and minority employees. Outside of continuity, they probably thought they were doing this tongue-in-cheekly by making her actually the only girl. Color me unimpressed.
Tex from Red vs. Blue. While she didn't balance out the sexes, throughout the whole show she has consistently been, by far, the biggest Badass of the series, at one point delivering a merciless smackdown against the Reds and Tucker at the same time without taking a hit.
Red Vs Blue usually avoids the negative aspects of the trope: particularly with Season 9, more and more female characters have been added, and they, as well as Tex, have been a major impact on the plot and have become well-developed characters. However, Sister's inclusion in Season 5 smacks of the downside: she's promiscuous, stupid, and female, and that's really all there is to her character. This may be why she hasn't been seen since the beginning of Season 6.
Arguably Zoe's inclusion into Sluggy Freelance (she's the first female character with any dialogue) after Riff comments, "This strip needs women." It's arguable because she appeared when the strip was only a month old, and Pete Abrams claims he'd had her character planned from the beginning.
Subversion; Pete dangled the implication of this trope across the Fourth Wall, but she was conceived from the beginning. However, the revelation that Aylee is female made her the Affirmative Action Girl.
The Order of the Stick begins with just one female, Haley (unless you count gender-indeterminate Vaarsuvius). In strip #43, they meet "The Linear Guild", their first set of antagonists, which includes two women, Sabine and Hilgya. Hilgya soon leaves, and Celia the air elemental law student is introduced. She goes on to become a much more important character, as does Sabine, who sticks around with her boyfriend Nale after the rest of the Linear Guild breaks up. Another female character is added in strip #200, the stuck-up paladin Miko. Xykon the lich's army starts out apparently all male, but gets an infusion of estrogen in the form of Tsukiko right before Miko lets her paranoia get the best of her and dies. Later we meet the Ascended Extra Kazumi Kato, and her boyfriend and eventually husband Daigo. The female paladin Lien also plays a supporting role. Intrigue among the exiled paladins and their people also introduces Lord Kubota as a villain, his female ninja protege Therkla, and the male imp Qarr. And it goes on from there. OOTS goes from Smurfette Principle beginnings to a nearly equally-balanced cast.
The second season of Avatar The Last Airbender. Until then, Katara was the only female in the main cast. Then Azula, Mai, Ty Lee and Toph all arrive in the first six episodes, and suddenly the gender ratio for main and recurring characters is four male, five female.
Though she appeared as a minor character in season one, Suki may also count as seasons two and three increase her prominence.
Incidentally, Toph and Azula were originally concieved as guys. The genders were swapped somewhere down the line, presumably to let this trope happen, and because swapping Toph's gender was hilarious.
Ellen Yin was the sole major female character for the first couple of seasons of The Batman. Unsurprisingly, she was written out of the show around the time Batgirl was introduced as a main character.
The show's version of the Terrible Trio had Vulture portrayed as a woman so that the group wouldn't be entirely male.
Skeeter in Muppet Babies, as Miss Piggy was the only major recurring female on the parent series, The Muppet Show. (For the record, there weren't very many minor recurring females on the Muppet Show, either. There was really just Janice, who was part of the rock band. On the other hand, Animal was part of the band, too, and they used him regardless.)
And Camilla (the chicken, and Gonzo's love interest).
Camilla also appeared in the cartoon as a stuffed doll that interacted with the other characters whenever they'd have their imagination sequences. Like when they did an episode on Greek mythology and needed a third female for the goddesses' contest with Paris.
The Geek in Sam And Max Freelance Police. Put in as a compromise - the executives wanted a female main character and their original idea was to make Max female (which would never work as Sam And Max are Heterosexual Life Partners with the occasional moment of Ho Yay). Luckily she hardly had any effect on the show. The silly thing is there are plenty of female characters in Sam And Max, but the network wanted one as a main character.
Kip Kangaroo in the second season of Shirt Tales.
Inversion: In the Animated Adaptation of WITCH, the producers added Blunk to increase the number of male characters (originally, they had only The One Guy in Caleb). Also in the second season, Matt (Will's boyfriend) was given a much bigger role, making them affirmative action guys.
Hawkgirl from Justice League was added because the network wanted one more female character besides Wonder Woman. It worked out, though, because despite her relative lack of notoriety in the original comics, she ended up becoming one of the most significant (in both a good way and a bad way, according to the Broken Base) in the series, and even led to her being used more in the mainstream DC universe, which in turn was because Timm and Dini are just that good. They are in fact so good that The Movie is entirely centered around her origins and race, and even when she leaves the team she's still a huge deal later on.
Dr. Hutchinson of Rocko's Modern Life was created originally for this purpose—the writers were told they needed "a professional woman [...] with a good hook." (They took that a bit more literally than intended.) Though reluctant at first, they eventually grew fond of her and created a storyline where she dates and marries Filbert.
And in Wacky Deli, a show-within-a-show from one particularly memorable episode, the three main characters each come up with their own characters for the show. Rocko's is introduced simply as "This here's Betty Bologna. She's a girl!". It is unlikely that he had any other plans for the character aside from being a token female.
For most of Reboot's run in the first two seasons, the main cast had a ratio of about 4-1. (Megabyte, Bob, Enzo and Phong to Dot.) Towards the end of the second season, likely to give Enzo somebody to talk to, AndrAIa the game sprite is introduced. Afterward, series guest stars Hexadecimal and Mouse are given larger roles and made into regulars.
Angela from Gargoyles, added in Season 2. All the gargoyles were male except for Demona, who was a villain. Elisa was the existing female character, but she's not a gargoyle and therefore not quite as action-y, despite being a police officer and often participating in combat.
In the original series pitch, there were apparently two female gargoyles in the main cast—Coco, who was forced by Executive Meddling to evolve into Broadway because they didn't like the idea of an overweight female as a main character, and Dakota, the original leader of the clan, who was dropped for being very boring. It's worth noting that neither character was stuck in limbo permanently: Coco was brought back in the comic continuation as a member of the London Clan, and Dakota went back to the drawing board and got retooled into a villain, under the name Demona.
Lola Bunny. Need more be said? Less so once they found a gender-neutral niche for her in the comic books.
Captain Scarlet's Lieutenant Green was turned into a woman for the CGI remake. Aside from a few shows of bravery and one episode in which the Mysterons cloned her father, her character was barely touched on.
Tron Uprising has Paige (the more competent of Tessler's Co-Dragons), Mara (one of Beck's co-workers), Lux, Pearl, and a guest appearance from Quorra. In less than a dozen episodes, they managed to have more female characters than two movies, ten games, two graphic novels, and the ARG combined.