The name of this trope comes from the Super Sentai/Power Rangers franchise, which has a yearly tradition of adding at least one new member to the initial roster of heroes during the course of each series. Specifically, the tradition was started by Kyoryu Sentai Zyuranger with the addition of Dragon Ranger, which was carried over by its adaptation Mighty Morphin' Power Rangers, where the introduction of the Green Ranger grabbed a lot of attention for the show and he became a Breakout Character. Almost every show in both franchises since then has featured at least one extra member.
Hikari Sentai Maskman featured a sixth member for exactly one episode in the form of X1 Mask, a warrior in green who volunteered for the Maskman project before the actual team was formed. Other shows had guest heroes who would assist the team for two or three episodes at most, but none of them ever became recurring characters (most notably Choudenshi Bioman had a two-part storyline about a man who wanted to become the sixth member of Bioman, but ended up becoming a brainwashedvillain-of-the-week, while Choujin Sentai Jetman had a birdman warrior who fought alongside the Jetman team before he was killed off).
MMPR is also a key example of just how much Popularity Power a Sixth Ranger can have. The trope-naming character, Tommy Oliver, is a mainstay in the Power Rangers setting, and he (and his actor, Jason David Frank) has been a regular for five seasons (two and a half of Mighty Morphin, Zeo, half of Turbo, and Dino Thunder) where the cast is now usually refreshed yearly. The only ones who even come close to such a record are Those Two Guys Bulk and Skull (all three seasons of Mighty Morphin, Zeo, Turbo, and in Space; plus Bulk's semiregular appearances in Lost Galaxy and Samurai). Big One is similarly popular after he completely stole the show in JAKQ, becoming one of the three possible candidates for being the Big Good of every Sentai team in existence.
Gekisou Sentai Carranger did not have an official sixth member; in fact, the show even parodied the concept with Radietta Fanbelt, a girl who dresses up as the White Racer, a self-appointed sixth member of the team. However, that did not stop Power Rangers Turbo from revamping the VRV Master, a supporting character in Carranger who appeared for a few episodes, into the Phantom Ranger. Because his Carranger counterpart was not a Ranger, there are those who also don't consider the Phantom Ranger a true Ranger despite his much-increased role and similarity to his predecessor the Gold Ranger, and feel that the sixth ranger role goes to the Blue Senturion (who was based on Signalman, the official extra hero in Carranger).
Invocation of this trope also created an interesting situation for the production staff of the Power Rangers Lightspeed Rescue. They wanted to carry on the tradition of the sixth ranger, but there was none in Rescue Sentai GoGoFive, the corresponding Super Sentai show.note The closest equivalent to a sixth ranger in Gogo-V was Jūma Hunter Zeek, an extraterrestrial warrior who assisted the team in a straight-to-video movie. He dies and passes on his powers to Sho's (Go Green) friend Kyoko, allowing her to transform into Zeek-Jeanne. However, Zeek was a one-shot character and Kyoko, despite being a recurring character, never used the Zeektector armor in the show. As a result, the staff created the Titanium Ranger from whole cloth and assigned him a robot that didn't belong to any of the rangers in GoGoFive.
The trope sometimes gets played with when a character has all the trappings of a Sixth Ranger, but doesn't join the team. The Gold Ranger from Zeo and the aforementioned Phantom Ranger were Mysterious Protectors that only dropped in to give aid on occasion, while the Magna Defender and Quantum Ranger were each an Aloof Ally to their resident teams, actively clashing with the Rangers over their goals. Even these were open to a Double Subversion; both the Gold Ranger and Magna Defender passed on their powers to people who did join the respective teams, while the Quantum Ranger eventually becomes a genuine ally.
Dekaranger, in fact, went even further. "Deka" is Japanese slang for 'detective,' but since the 'deca-' prefix means ten of something, they went ahead and had ten Rangers total with the aforementioned seven full-time teammates and three One Shot Characters: DekaSwan (Mission Control getting to suit up for once), DekaBright (a Distaff Counterpart to DekaBreak, one of the regulars), and DekaGold (a movie-exclusive character who barely gets to do an Ass Kicking Pose before being blasted). SPD carried over the first two as the Cat Ranger and Nova Ranger, respectively.
Bakuryuu Sentai Abaranger had not only two additions (Abare Black and Abare Killer), but AbaRed also got a Super Mode called Abare Max. The writers of Power Rangers Dino Thunder, which was under production while Abaranger was starting to air in Japan, planned to turn Abare Max into an actual sixth team member known as the Triassic Ranger until Abare Max's debut episode revealed that it was just a powerup for the Red Ranger.
Power Rangers Jungle Fury added the three Spirit Rangers to the original roster of Three Plus Two from Juken Sentai Gekiranger. Gekiranger already had three mentor characters who had weapons and mecha associated with them, so Jungle Fury eventually gave them Ranger suits as well in order to create action figures of them. They're unique in that either the Rangers or the mentors can project them at a distance; made from spiritual energy like the Zords of the series, nobody even needs to be physically wearing the suits. The Rangers often summoned the Spirit Rangers from their own power to fight without the mentors needing to be present, and in the finale the mentors fought alongside their own Ranger forms, awesomely.
Kaizoku Sentai Gokaiger, keeping with its team's premise of transforming into any of the previous 34 Super Sentai teams, features a sixth member named Gokai Silver, who wields the Ranger Keys of fifteen previous "supplemental warriors." note Although he can use other Ranger Keys as well He has the ability to merge all of his Ranger Keys into the Gold Anchor Key, which allows him to transform into his "Gold Mode," where he wears a golden armor adorned with the faces of said warriors.
Super Sentai Battle Dice O makes this into a playing feature. Players assemble a team of five Rangers, one robot, and an EX Card that can either be a one-round addition to the team or a team-wide special move. On the other hand, sixth / extra characters can still be part of the main five.
In Sailor Moon, first Chibiusa, and later, Uranus, Neptune, Pluto, and Saturn, and still later, the addition of Chibichibimoon and the Starlights.
The Outer Senshi provide egregious examples. They are somewhat xenophobic but dissappear for vague reasons whenever it'd be tricky to have overpowered characters the next season around. Whether or not they become weaker is moot, since last season sees them as Cannon Fodder. Another example is Mamoru, as he joins before any girl other than Usagi.
Jokingly Deconstructed in Samurai Pizza Cats, where a set of proud Sixth Rangers is snidely narrated as the "B-team", who fight villains that main crew are too busy or can't be bothered to fight.
Done over and over in Mermaid Melody Pichi Pichi Pitch. Lucia, Hanon and Rina are a Power Trio throughout most of the series, but the plot of the first season is dedicated to finding the other four girls needed for a whole team... one of whom is a rogue fighting both the good and bad guys, while another is The Man Behind the Man (well, the woman behind... oh, whatever).
At least one every Digimon season, the first of which is always heavily hinted at in the twenty-first episode but introduced an arc later. But it comes as the result of a Heel-Face Turn every time.
Suite Pretty Cure ♪ introduced Cure Muse early on, although she doesn't join the team, let alone reveal her identity until much later in the series, where she does both. Later, it followed Fresh's example and added a reformed Dark Magical Girl of its own: Cure Beat, who joins the team before Cure Muse.
Happiness Charge Pretty Cure has Third Ranger Cure Honey and Fourth Ranger Cure Fortune. Like Moonlight and Muse, Fortune appears early in the series but doesn't join until later.
Wedding Peach, originally featured a Power Trio consisting of Wedding Peach, Angel Daisy, and Angel Lily. But in the second season, a "Sixth Ranger" is added in the form of Angel Salvia, a mysterious character with a modified costume who wields a sword, is much more mature and powerful, and frequently acts independently of the main trio.
Ghost Hunt has two examples of this. The first is Lin Koujo, who was injured in the first episode and then left out to heal until the end of episode three. The second is Yasuhara Osamu, who shows up in episode 14. These two characters also follow the 'power is inversely proportional to the number of episodes in' rule mentioned at the top of this page: Lin is easily the strongest character in the series excluding The Hero Shibuya Kazuya, but he doesn't show his power until the final episode, while Yasuhara has absolutely no power whatsoever and does all of the research for the group.
In The Melancholy of Haruhi Suzumiya, Tsuruya, for the Five-Man BandSOS-Brigade is very involved in their activities, but for some reason isn't recruited. That is, despite being a natural co-conspirator of Haruhi, since they are so similar. She also appears to know that the SOS-Brigade isn't "normal", telling them to work on their masquerade more. She actually states that she prefers looking from outside, though other people on the outside sometimes considers her part of the group.
Schwarz Bruder and Allenby Beardsley from G Gundam. Super Robot Wars Reversal ran with this idea towards Allenby, especially as the game seemed to have portrayed the Shuffle Alliance as a Super Sentai-esque team.
Zechs from Gundam Wing is the Sixth Ranger when he's on the Gundam Pilots' side and The Rival when he's not. Appropriately enough, his name is derived from the German word for "six", fitting in with the series' Theme Naming.
Shitenhouji: Chitose Senri transferred to the school late (in fact, he was never supposed to even be part of the team, but the author changed this to accomodate the story, and in so doing pushed the team's vice-captain into almost complete irrelevancy), and in many ways remains a loner
Hyoutei: Hiyoshi Wakashi is a loner, initially held as a reserve. He would like to change the status quo but is not able to do so. In some way he is the Token Evil Teammate
Rikkai: Kirihara Akaya is the lone second year among a team of third years. From the start he does aim to shake the status quo. He's actually simply assimilated.
Seigaku: Echizen Ryoma starts out as a Sixth Ranger and becomes Designated Hero
Soldat J and King J-Der from GaoGaiGar fit pretty well. In his first appearance, he shows up out of nowhere and massacres the three Primevals that had just annihilated the hero's main base and, it appeared at the time, the entire main cast aside from the hero himself. While Soldat J never truly joined the heroes, coinciding goals caused him to fight alongside them all the way to the end of the series and OVA. Interestingly, while the main protagonists did somewhat catch up to him, he was still more powerful than any other major character in the series except for the main character himself.
Voltron has two that could be considered sixth rangers : Either Princess Fala/Allura or Sven/The Shirogane Brothers. In the twenty episode second season of Voltron, made not from Go Lion but new animation, Sven actually gets one last chance to pilot Blue Lion before the second finale.
The Slayers TV series features frequently this trope:
At the beginning of the series, Lina and Gourry form a duo and Zelgadis is an enemy, then later fighting against both sides, and finally settles in as an ally.
By the beginning of the second season, they've established themselves as a standard four-man RPG team, but Martina keeps pursuing them and trying to curse Lina, before finally more-or-less joining them near Katahto.
Borderline example: Filia attacks the team at the beginning of the third season, but she's joined by the second episode.
If you count only his third-season appearances, Xellos starts out pranking them, then joins them to fight the apparent villain, then switches sides a half-dozen times before finally joining them for the last couple of episodes.
In the Hourglass of Falces manga, the main four from the anime are also joined by Luke and Milina, who serve as Zelgadis and Amelia's replacements in the second half of the novels. Luke acts as Lina's Lancer in this story, thus the snarky, well-rounded fighter Milina fits this trope.
Most of Goku's allies in Dragon Ball started off as adversaries. Yamcha was a desert bandit that fought with him over the Dragonballs, and Tenshinhan, Piccolo, Vegeta, Android 18, and Mr. Buu were all trying to kill Goku at some point.
Mari plays this trope straight in Rebuild of Evangelion: she scores a solo kill in her debut battle and actually survives. Later on, she hijacks Unit 02 and goes up against Zeruel, putting on a futile but otherwise awesome fight. Kaworu also appears to head into this territory: he appears when there are only two angels left and his first act is oneshotting Unit 01 with a lance. Unit 01 has just turned intoa GOD. And Kaworu incapacitates it within seconds.
Also Fe-chan, Weboshi, and Mihoshi. The first two were present in the background, but didn't become recurring characters until about halfway though the series' run, while Mihoshi doesn't even show up until episode 18. Like Nano, the three girls were added to the opening credits right around the time they joined the cast.
Lucky Star had a whole slew of sixth rangers that joined the cast about halfway through the show's 24-episode run. Misao and Ayano start off as Living Props before eventually becoming recurring characters, while Yutaka, Minami, Patty, and Hiyori don't show up until the new school year starts. Humorously, Misao Lampshades this by pointing out that she and Ayano had been there from the beginning, but were simply Out of Focus.
Taki of Eyeshield 21, who becomes the tight end for the Devil Bats. He joins during the middle of the Death March through America and is eventually Flanderized into becoming a huge idiot and an extra person to make short passes to. Only on one instance was he a winning factor for the team.
Musashi even more so. His existence has been alluded to since chapter 1, first appeared in volume 5 as a nameless contractor, and had his identity revealed in volume 7. He appears to be a jerkass at first, but it's soon revealed to be a facade and he's actually a really good guy. He doesn't join the team till volume 18, and his importance to the team only being hyped up during his first two match. Afterward, we rarely even get to see his kicks and his role in the story significantly decreases.
Axis Powers Hetalia has Canada, to the point that no one ever notices him or mistakens him for America.
Ikki from Saint Seiya is the quintessential Sixth Ranger... despite being the fifth member of the core team.
Yu-Gi-Oh! 5Ds had Crow become the Fifth Signer, while Rua/Leo became the Sixth Signer with about a dozen episodes left to the end of the series, making the latter more of an Eleventh Hour Ranger.
Martian Successor Nadesico had Loads and Loads of Characters as it was, but continued to pick up more as the series progressed. First was the three spare Aestivalis pilots, Ryoko, Izumi, and Hikaru. After they returned to Earth, they reluctantly allowed military-appointed leader Admiral Munatake, the company chairman's secretary Erina, and fifth pilot Akatsuke Nagare the chairman himself. Finally after the exiled crew's hijacking of the Nadesico, the Jovian pilot Shiratori briefly joined.
Hana in the 4th season of Ojamajo Doremi. She transformed from her baby form to her human form to become the same age as the girls. Before that, Onpu was the Sixth Ranger to the original Power Trio, though she was fully integrated into the team in Sharp.
Lupin III started out with the main cast being just Lupin's friend Jigen and the stranger Fujiko. The samurai Goemon was added later on because the creator thought there should be a clearly Japanese character. True to the trope's usual form, he was a bad guy at first - an assassin sent to kill Lupin - but they soon became allies instead.
Cyber Team in Akihabara started with Hibari, Suzume, and Tsugumi as a Power Trio. About halfway in, Kamome shows up and acts antagonistic, but forces her way into the team somewhat awkwardly. In the second half, Tsubame is introduced as a villain and eventually Heel Face Turns close to the end of the series.
Fairy Tail has Gajeel, who started off as an Evil Counterpart to Natsu until he joined the guild, whereupon he became The Rival, and is still not trusted by some members of the guild. He is, however, a Double Agent for Makarov and takes on the tough job of spying on Raven Tail for him. There's also Wendy Marvel and her sidekick Carla, who fills the temporary 6th character spot in their intro arc, but proceeded to become a permanent addition to both the guild, and the usual focused team of Natsu/Lucy/Gray/Erza/Happy.
Bakura, moreso in the manga than the anime, who joins the core four of Yugi, Jonouchi, Honda, and Anzu. He notably has this role for most of the manga, while in the anime it only really lasts for the Duelist Kingdom arc. If anything, for the majority of the anime, Kaiba actually somewhat fills the role moreso than anyone else.
Despite having been there from the start, Pharaoh Atem himself is this to an extent; he and Yugi only really identify as entirely separate characters to each other. Ishizu and all the major antagonists (except Pegasus) act as if there is only Atem, while everyone else acts as if there is only Yugi.
The online RPG game places this situation on Homura in regards to the pre-existing Magical Girl team of Madoka, Mami, Sayaka, Kyoko, and the Player Character. The combination of Homura's newness to the rest of the group (beyond just her New Transfer Student status), her unsocialbility and aloofness, and her choice of firearms and explosives presenting an ever-present risk of Unfriendly Fire on melee-fighters Sayaka and Kyoko make her integration into the group difficult - the later chapters of the main storyline place more focus on this.
Renji Abarai is the most obvious example in Bleach, becoming committed to fighting alongside sworn enemy Ichigo and his close pals after getting defeated by him in the Soul Society, and succeeding in rescuing Rukia together. Renji's still an abrasive guy but likes chilling around with Ichigo whenever nearby, especially in the anime's numerous filler arcs.
Pokémon has a whole horde of recurring secondary characters who join the main three or four characters for specific episodes or arcs. Often, they will be actual characters from the games. A non-exhaustive list could include Todd, Richie, Professor Birch, Drew, Morrison, Barry, Zoe, Kenny, Cynthia, Nando, Angie, Bianca, Stefan and Luke. Misty, May and Dawn also act somewhat like Sixth Rangers in their return appearances. Also add Larvitar and Meleotta as pokemon examples that never quite joined the group.
Back when the X-Men were a Five-Man Band, Mimic was the first new student at Xavier's (also the first non-mutant X-Man). He's been a low-level criminal since leaving the team (a waste of potential after seeing his Exiles counterpart in action).
When the original X-Men "graduate" to make room for the All-New, All-Different team, the lineup remains fairly constant...Wolverine, Storm, Nightcrawler, Colossus, and Cyclops, with Banshee or Phoenix rounding out the squad on occasions...until after the Dark Phoenix Saga, when the team picks up Tagalong Kid Kitty "Sprite" Pryde. In an interesting inversion, she started off without much combat effectiveness, and took several levels in badass as her time with the team progressed.
Victor Mancha of Runaways, as well as Xavin and Klara Prast.
About a fourth of the the way into its run, Teenage Mutant Ninja Turtles Adventures, the Archie-published book based on the first animated series introduced Ninjara, a ninja foxwoman, as an antagonist. A few issues later, she joins the turtles and becomes a series regular, appearing in every story since.
In the original Mirage series (and most of the adaptations) this role is usually filled by Casey Jones who first met the Turtles after getting into a fight with Raphael when the latter stopped him from almost beating some street hoods to death.
The Fantastic Four have had a few extras over the years—Johnny's ex-wife Lyja, Ben's girlfriend Sharon Ventura, the time displaced teenage version of Valeria Richards, and others; not counting substitute members who have filled in temporary vacancies in the team like Crystal, Luke Cage, She-Hulk, and the Black Panther.
The second was Tenebris, Darkhell's daughter, who joined the group in order to protect her love interest Razzia. Though her team-up with them was initially somehow intended to be temporary, she ends up joining them for real after Danael was possessed by God of EvilAnathos and became the new villain. Ironically enough, Shimy was so much expecting Tenebris to be a Sixth Ranger Traitor that she was willing to kill her in order to prevent it from happening, when in fact it turned out Tenebris remained loyal to them.
Archie Comics' Sonic the Hedgehog has had a number of Sixth Rangers since it's inception: Bunnie Rabbot (who wasn't part of the team here), Dulcy the Dragon, Tails and Amy Rose. The Cosmic Reboot altered things so that Big the Cat and Cream and Cheese are now Freedom Fighters.
Tsuruya was officially recruited into the SOS Brigade in Kyon Big Damn Hero, as well as the unnamed girl from Dissociation.
Trixie serves this role, being a second Element of Magic and performing a Heel-Face Turn when Twilight saved her from her Discording and an Enemy Within named Loneliness, joining the group afterwards. She proves to be a powerful ally, not only sharing Twilight's Power Copying ability, but by being a cunning Guile Heroine who makes good use of her weather generating abilities.
Fluttercruel also ends up joining the team, after she pulls a Heel-Face Turn, nearly performs a Heroic Sacrifice to defeat Nightmare Whisper, is discovered to be a second Element of Kindness, and she and Fluttershy are given the ability to swap control of their body by Celestia.
Also in Dark World, there's a villainous example, as Discord's sisterRancor joins up with the villains just as the Elements of Harmony start their campaign to liberate Equestria from Discord. Becomes a Sixth Ranger Traitor, as it turns out she was only bidding her time until she could steal Destruction's power from Discord for herself.
Rainbow Dash gets demoted to this — unlike the other Mane Six, she goes into a Deep Sleep after her redemption in order to recover spiritually from the trauma of everything Discord put her through. As such, she doesn't join in the fight against Discord's forces until she shows up in time to save the others from Odyne!Cruelty. Likewise Pinkie Pie, who isn't redeemed until close to the end of the arc and dies soon after anyway.
Shining Armor's side story has a rather literal example — Cadence has Private Garnet reassigned to Misfit Actual (Shining's unit) not just for her medical skills, but because regulations dictate that units need to have six members.
Silver Spoon ends up joining the Cutie Mark Crusaders, and is rather determined to make former friend Diamond Tiara join them too.
On the subject of My Little Pony: Friendship Is Magic fanworks, its pretty common to depict background filly Dinky (who is usually depicted as the daughter of Memetic Bystander Derpy Hooves) as the Fourth Cutie Mark Crusader. Similarly, it's also quite common to have Trixie ultimately become a seventh member of the Mane Six.
Rainbooms and Royalty: Zecora joins the Mane Six and guides them through the Everfree forest to find the Elements of Harmony. Ditzy Doo, after being freed from Nightmare Moon's mind control, likewise joins during the Final Battle.
Zecora seems set up to take the role again in Hot Heads, Cold Hearts, Nerves of Steel.
Fate Revelation Online: Shirou is explicitly nicknamed this, due to his rampant heroism in the early days of SAO. He even gets an appropriate white jumpsuit and mask made for him.
Breeze Rider from Dusk's Dawn is generally comic relief throughout the episode.
Equestrias First Human: When Connor permanently ends up in Equestria, he is reincarnated as a pony, and joins the Mane Cast in it and the rest of the sequels. While the sequels are written from his perspective, they take place during canon events of the show which tend to focus on the Mane 6, and thus he qualifies.
Star Wars: Lando Calrissian. He double-crosses the heroes in The Empire Strikes Back, and Chewbacca nearly strangles him to death. However, he was just trying to protect Cloud City, and he eventually pulls a Heel-Face Turn to join the Rebellion.
Captain Barbossa of Pirates of the Caribbean, who becomes a good guy in At World's End (until the end, when he steals the Black Pearl from Jack...again). A more minor example might be Jack Sparrow, who joins up with Will Turner after them being quite at odds.
Stagecoach' - The Ringo Kid. John Wayne's character in the classic western is introduced after the other characters and joins them later in the story. With the characters opposed to each other, he acts as a catalyst to bring the other characters into reconciliation.
The Ghost Busters where originally a Power Trio in formation, then Winson Zeddmore joins immediately after the Montage. In the sequel he also oddly appears in only 1 Pre-montage scene.
Kung Fu Panda - After Po proves himself as the true Dragon Warrior in the first film, he regularly fights with the Furious Five. However, considering he is still unsure of himself to a certain degree, he defers to Tigress as leader and is content to keep the relationship known as "The Furious Five and the Dragon Warrior."
Transformers: Dark of the Moon - After the Autobots learn of an Autobot starship hidden on the moon's dark side, they go up to retrieve its captain, Sentinel Prime. Sentinel then joins the Autobots at NEST, before revealing himself to be a traitor. Unknown to the others, he had struck a deal with Megatron near the war's end to work together rebuilding Cybertron. He then murders Ironhide, before taking the Space Bridge pillars and meeting up with Megatron, with the intention of bringing Cybertron close to Earth, and using humanity to rebuild Cybertron as a slave labor force.
In The Magnificent Seven there is a bit of this going on in two ways. The character Chico is viewed as this by the other characters, although the character Lee, played by Robert Vaughn, unlike the others, does not have a counterpart in the Seven Samurai and was created specifically for the film, making him also a Sixth Ranger.
David from Animorphs is the straight example, identified on book covers as the "Sixth Animorph", though he is actually a Sixth Ranger Traitor. Ax is the more commonly accepted Sixth Ranger: he and the original five initially consider themselves "five kids and an Andalite," but by the final book, Ax refers to the group as "We, the Animorphs," when speaking to Jake.
Starting in Harry Potter and the Order of the Phoenix, Neville, Luna, and Ginny (all of whom but Luna had been recurring characters before) began to function as a sort of auxiliary trio to the main characters. In Books 4 and 5, the role is largely filled by Sirius.
Thomas from Malevil. He's the one member of the World War III survivors that wasn't part of the group from their youth. He's also college educated, don't speak the local patois, and an atheist without a religious upbringing.
There are some borderline examples in the first Dinoverse in Loki and Beanie. Loki never really joins so much as he comes and goes, mostly overseeing Janine. Beanie shows up late and sticks right to Bertram's side. There is a much clearer example in the second set in Green Knight/GK, who in his first appearance tries to eat Zane. (It should be noted that GK is a predatory dinosaur.) GK's love for Patience is strong enough that he follows her around and refrains from eating her friends, and in fact tries to save one of them in the climax.
Live Action TV
Mighty Moshin Emo Rangers, a UK fan-made series (that made its way to MTV) which parodies both the Power Rangers and "emo" culture, did this in its first season with the addition of Fai, the Purple Fashioncore Ranger.
In season 4, Buffy herself becomes the Sixth Ranger to The Initiative, the newly-introduced organization staffed by red shirts guest-stars.
During season 5, the Knights of Byzantium start off as possibly ally or enemy (attack Buffy, but also beat up Glory's Mooks) before becoming full-on enemies due to their unwillingess to show mercy.
Spike Started off as a Big Bad in Season 2, later performed a Heel-Face Turn and did indeed end up as the protagonist's boyfriend.
Anya starts out bad, but performs a prolonged Heel-Face Turn, and ends up part of the group.
And finally Andrew, who spent most of season 6 fighting against Buffy, only to end up becoming one of the Scoobies the following season.
Inverted during Season 3. Faith starts out good and ends up good, so she is not only Inverted, but also a true Sixth Ranger, two different times. Three times if you include her short stint in the fourth season of Angel. Faith serves as a sort of wild card for either show when the dynamic needs a good shaking.
Lorne fits this trope in the Buffy spinoff Angel. While he debuts in season 2 as a neutral contact when the team has 4 members (Angel, Cordy, Wes, Gunn), he moves in with them and becomes far more involved with them in season 3 when they have 5 members (the previously mentioned, plus Fred) then joins the team officially in Season 4. But during season 3, while he is considered an ally, he isn't considered a core member. After Cordy departs, and Lorne makes up the fifth member, it could be said that Spike becomes the new sixth ranger.
Daniel Shaw from season 3 of NBC's Chuck. He occasionally joins the Power Trio of Chuck, Sarah, and Casey on missions but only those concerning his area of expertise, the evil organization known as the Ring.
Oddly, Iron Chef has a one of these: Kobe in the original series fits the trope almost perfectly (as he even has his own customised entrance).
Kylie Minogue was once designated as the pink Wiggle.
Richard Grieco's character, Dennis Booker, on the crime drama, 21 Jump Street. His character was then given his own short-lived spinoff.
Nasir silently joins the Merry Men at the end of the pilot episode of Robin of Sherwood after being set up as a minion of the bad guys. Yes, that's one episode in, but it's also a brand new Robin Hood character after about 600 years.
Supernatural has seen several attempts at adding a Sixth Ranger to the Winchester brothers, with varying degrees of success. Ruby turned out to be a Sixth Ranger Traitor, but Castiel and Bobby seem to have well and truly worked their way into Team Free Will.
Friends: Charlie Wheeler, who happened to be a Twofer Token Minority. Also Richard and Pete, two of Monica's early boyfriends that got their own arcs. Other examples include Mike Hannigan, who marries Phoebe in the end; Fun Bobby, Marcel the Monkey, and Gunther who appeared in the most episodes after the main six.
Double The Fist has Tina T, The Chick who replaced Mephisto after he was taken over by an Aztec Demigod. In Series Two we got Tara, a shapeshifting cyborg assassin who was on par with Steve in combat. Comically, she was mostly used for paperwork. Blue Womp would also technically count.
Jack Harkness fits this trope in the 2000's series, temporarily changing the Doctor and Rose's The Hero / Side Kick duo to a Power Trio (once he stops trying to con them).
Series 5 has Rory to Eleven and Amy. He's a bit character when he first appears, but eventually becomes a companion midway through it. And then he dies and is blinked out of existence a few episodes later, only to return for the finale. And then by Series 6 he becomes a full-time companion for the rest of Amy's tenure.
For the first half of Season 1 or so, this is Ianto, but by the end of the season he's just as much a part of the gang as the others. Season 2 has Martha and Season 3 has Lois.
Rhys is the recurring Sixth Ranger from the middle of Season 2 onwards.
On Glee, later additions to the club have included Rachel's boyfriend Jesse from a rival glee club in the first season, new student Sam and Deadpan Snarker Lauren in the second, and Irish exchange student Rory and spoiled rich girl Sugar in the third.
Vala Mal Doran in Stargate SG-1. In mid-season eight, she unsuccessfully hijacks the Prometheus. In Season nine, she gradually moves from minor villain to anti-hero, being partially responsible for bringing the Ori to the Milky Way among other things. In season nine, she becomes a full SG-1 member, though not after losing her memory once.
Played with on episode 2.4, which introduces a guy named Ollie with all the traits of a sixth ranger: he joins the community service with the main gang, starts to get along with them and even has a superpower like them... but he's killed ten minutes into the episode. Ironically, Ollie's heart is transplanted to Ill Girl Nikki, who eventually becomes the sixth ranger for the rest of Season 2.
Gunther and Tinka Hessenheffer in Shake It Up, despite the latter being billed as a recurring character.
In the second season of Community Chang attempts to join the study group.
Some of the shows in the Ultra Series have featured secondary Ultrabeings that help out the main hero in times of need, although most of them were just guest characters that appeared for a few episodes or heroes from previous shows. It wasn't until Ultraman Gaia that the show featured two regular Ultra Heroes, the eponymous Gaia and his rival Ultraman Agul.
Done multiple times on Babylon 5. Most of the time, it was to replace a departing character (Sheridan for Sinclair, Lochley for Ivanova), but Marcus Cole, introduced in the third season, is a straight example.
Kat of Alphas was introduced in the second season and spent the majority of the following episodes trying to officially join the team, which she eventually succeeded in doing.
In It's Always Sunny in Philadelphia, Frank appears and joins as the fifth member of "The Gang," which had previously consisted only of Dennis, Dee, Charlie and Mac. Frank has become a staple of the show.
Hawkes in CSI NY, who joined the team as a field investigator in season 2. He was coroner before.
Jerome from House of Anubis who technically joins Sibuna for protection from his "boss" Rufus Zeno. Joins up with Sibuna again in Season 2 So they could rescue Trudy who had been kidnapped by Rufus.
Joy also counts as she was important to the mystery in the first season and was added into Sibuna by the end of season 2 after helping to rescue Nina from under the Senet board. In season 3 she is a full member of the team, but she quits after Fabian breaks her heart.
Bob Fossil of The Mighty Boosh is a sort-of Fifth Ranger to the main group of Howard, Vince, Bollo and Naboo. He was in the radio show and tv show from the beginning, but as antagonist with not much focus. After series one ended, he vanished from the show, as the main characters no longer worked for him. He had a single cameo in series two, then in series three he appeared in a neutral role in a few episodes, though the other characters still don't like him. In the live shows, however, he's updated to full group status, fights on the side of the heroes, and seems to be at least somewhat friendly with Vince and Howard.
Iron Maiden had five men line-ups since 1980. Then when Bruce Dickinson and Adrian Smith decided to return to the band, Janick Gers served as a Sixth Ranger to a reunion of the golden years Maiden. This meant that the band had THREE GUITARISTS and is one of the few (and certainly the most popular) band to do so. It is said that Janick was kept on for three reasons - he had worked with Bruce solo, he kept the group entertained, and so that Adrian and Dave able to do twin solos and have a rhythm guitarist underneath.
The Black Eyed Peas were originally a trio (the three guys), before Fergie joined.
Keyboard player Billy Preston joined The Beatles for their Let It Be album/movie in 1969, was the only musician after 1963 to be co-credited on a Beatles single. and was commonly called the "Fifth Beatle". George Harrison, who brought Preston into the studio in the first place, unsuccessfully tried to convince the band to make him an official member, noticing the positive effects Preston's presence had on the band on the band musically and personally.
Other candidates for the role of "Fifth Beatle" include their producer George Martin, and sometimes their manager Brian Epstein. Others who were involved with them are ex-drummer Pete Best, ex-bassist Stuart Sutcliffe (who quit the band to get married before they made it big, then died of an undiagnosed aneurysm a year later), and Eric Clapton (he played on several of their tracks and was seriously considered for membership in the band at one point).
Raymond Watts had this effect when he briefly rejoined KMFDM for their album Nihil. The usual vocal lineup of Sacha K, En Esch, and whatever female singer they were working with at the moment (typically Dorona Alberti) was disrupted as Watts took over lead vocal duties on half of the album's tracks and En Esch was only the featured singer on one song. This paved the way for the band to become more reliant on guest singers for the next four albums with Watts, Chris Connelly, Ogre and Tim Skold (who was also something of a Sixth Ranger himself) appearing on multiple tracks.
Linkin Park's bassist Dave "Phoenix" Farrell. He was a part of the original lineup, known as Xero from 1996-1998, but had commitments with another band called Tasty Snax, so left the band until 2001, when he returned. During the period he was away, the band recorded their first album Hybrid Theory and got their first mainstream exposure, which brought them great popularity. On the back cover of this album, only the five main members are shown, which led many to assume those were all the people in the band. On the Hybrid Theory album itself, bass duties were split between usual guitarist Brad Delson, touring bassist Scott Koziol and session bassist Ian Hornbeck. played the rest. When Phoenix returned in 2001 shortly after the release of One Step Closer as a single, many fans who didn't know about him felt that he was the Sixth Ranger joining an established band, and to many it still feels like that.
Eddie Vedder of Pearl Jam. Pearl Jam developed from Stone Gossard and Jeff Ament, the surviving members of Mother Love Bone, who had been one of the founding members of the Seattle grunge scene. They recruited Eddie after lead singer Andrew Wood's death, and for a substantial period of their early career, Eddie Vedder was seen by many as having taken the place of his predecessor, despite the name change.
Swedish Garage Rock quintet The Hives credit their manager Randy Fitzsimmons as their sixth member and main songwriter. However, Fitzsimmons may or may not actually exist. The Hives claim he is a real person. The music press are deeply skeptical.
Yellow Magic Orchestra have always been primarily thought of as a trio, but several of their collaborators (such as Chris Mosdell and Peter Barakan, who wrote most of the band's English lyrics) have been unofficially considered the band's "fourth member". The primary candidate, however, is Hideki Matsutake, who produced most of the band's early releases and has actually performed as part of their touring lineup.
Played straight in many series of BIONICLE. Beginning in 2003 (and ending in 2009), Lego sold an extra basic-sized figure (usually referred to as a "Seventh Toa") packed in with a vehicle and sold at a higher price point, and usually joined the main team of heroes in-universe later in the story, though almost every year deviated from this in some way;
2003 - Takanuva was a new member who joined the team just before the finale of the first movie (and stayed with them afterwards) and was packed with the Ussanui (which existed for about 30 seconds before being destroyed in the movie) and is considered the basis for the seventh toa in the rest of the franchise.
2004 - Toa Lhikan was packed in with a Kikanalo, though in-universe, he existed before the main heroes and is the one who gave Toa Metru their powers.
2005 - The only year without a true Seventh Toa, though the special edition "Toa Norik" and Toa Iruni were sold on the side and fill the seventh toa slot (though both were sold at the same pricepoint at the regular figures, neither had a vehicle, and were completely disconnected from the main story in-universe).
2006 - Vezon filled the seventh toa slot but was a villian instead of a hero. Vezon was packed with the mechanical spider Fenrakk, and (in-universe) was a clone of the Piraka Vezok (Vezon is even the Matoran word for "Double") and fought both the Piraka and the Toa Inika.
2007 - Lesovikk filled the seventh toa role toy-wise (being packed with a "Sea Sled"), but never met the main heroes (the Toa Mahri) in-universe.
2008 - Toa Ignika, packed with a "Skyboard" was, in-universe, the Mask of Life given a physical body who joined and fought beside the team after initially being questioned.
Ironically, the original Seventh Toa, Takanuva, also made an appearence (though in a much larger, Titian-sized form).
It should be noted that there was another villian, Murtan, who was also a basic figure and was packed with another smaller figure (Vican) and was sold as an exclisive, though Ignika more closely follows the Seventh Toa formula.
2009 - It could be argued that Fero falls into the seventh toa slot as he was a basic figure (though he was matoran sized, not toa-sized) and was packed with a large reptilian steed, Skirmix.
Chrono Trigger - Magus (optionally) joins the party very late into the game, significantly less powerful than he is when you fight him (either at the halfway point of the game or if you chose to fight and kill him there instead of have him join you). He very much has an 'odd man out' feel to him in other ways as well (he has no 2-person combination attacks and very few three-person combination attacks, the initial six characters have 3 2-person combination attacks for each of the five other characters among them, and there is one 3-man combo attack for each mix of Chrono + 2 of the other first six.)
Most JRPGs have a section where the party is exactly 3 (or 4, or 5, whatever the max party size) is members.. with characters leaving and being conspicuously replaced, until suddenly you get a 4th (or 5th, or 6th) ranger, and every point after that you can pick your own party (often having the other characters who left earlier come back)
A few games buck the trend in an unexpected way by never actually giving you a permanently set last member. Final Fantasy II continues shifting out the 4th party member without the player's control straight up until the final dungeon, at which point one of the main villains joins your team.
Meanwhile, some other games (such as Final Fantasy XII and Tactics) have a party slot dedicated to whatever NPC you'll be dragging around until you hit the next plot point, and leave you a man down the rest of the time.
Golden Sun: The Lost Age has an interesting variant - the heroes from the first game will join the party as Fifth through Eighth Rangers after a certain point in the story.
Iori Yagami in The King of Fighters, who was initially introduced as Kyo's rival (to the point that Iori was trying to kill him most of the time), and then became his reluctant ally when Orochi showed up.
Final Fantasy VII and Final Fantasy VI have a few secret characters each that don't contribute to the plot at all, but are so well known and easy to obtain that they're normally regarded as sixth ranger-esque members of the main cast.
FFVII specifically has Cait Sith, who is recruited into the party after the majority of the main cast (although he is actually the 6th party member after Cloud, Tifa, Barret, Red XIII and Aerith not counting players who recruit Yuffie 'early') but does almost nothing untill he is revealed to actually be a bad guy who then turns to the good guys side for real later on
In Super Robot Wars W, the Golion team actually asks Guy of GaoGaiGar, an old friend of theirs from the space academy, if he wants to be their Sixth Ranger. Although he politely declines, he occasionally ends up playing the role anyway, especially considering that in this game, both Ghaleon and Golion have the same origins. The game actually references the Sixth Ranger trope a bit; when Guy points out that Ghaleon can't combine with Golion, Kogane comments that the sixth member of such teams typically has a special Humongous Mecha that tends to overshadow the other five in terms of power.
The GoLion team helps out Guy, too. During the battle between Guy's GaoFighGar and an enemy repli-GaoGaiGar in the FINAL storyline, another Replijin of GaoGaiGar's own Sixth Ranger, King J-Der, shows up to make things worse... at which point the Hundred Beasts King appears to even the odds.
In the Original Generation series, the crew of the Kurogane is a team of Sixth Rangers. In the first two games, it was made up of reformed AntiVillains and Stealth Mentors, and they consider it their duty to operate independently until it's time to join the main cast halfway through the latest game's plot. The leaders of the crew, Sanger and Elzam, could also each be traited as a Sixth Ranger in his own right to one of the protagonists' sub-teams (Sanger to the ATX team, Elzam to the SRX team).
In Tales of the Abyss, Asch fits this role perfectly: he's a powerful antihero with ambiguous motives who joins the party for a while (and is much more powerful than the person he replaces). Later you get a second chance to recruit him into the party, only this time he's way behind the power curve, due to being stuck with the same equipment he had twenty levels ago. This is Lampshaded with the Abyss Rangers costumes you can acquire: Asch becomes "Abyss Silver", a typical Sixth Ranger colour in Super Sentai.
In Lunar 2: Eternal Blue, the party is in flux for most of the early part of the game, gradually picking up additional characters, eventually stabilizing on 5 members (not including Lucia, who is not always player-controlable, and is occasionally abducted and must be rescued by the party). One of the antagonists, Leo, joins then leaves the party several times as a Sixth Ranger, both as himself, and as his blatantly obvious alter-ego "Mystere" (the absurdity of his disguise is lampshaded by the other characters).
Depending on how you played the game, you probably went through the majority of Devil Survivor with only 3-5 party members. Cue the final day when the rest of the supporting cast get tired of being NPCs. Depending on the decisions you've made throughout the game, it is entirely possible for your party roster to double in size.
In Persona 4, Naoto is the last character to join. Given that Naoto is also an actual Detective rather than an Amateur Sleuth like the rest of the Investigation Team, it also marks the point where they are finally able to make some headway into the murder cases.
In Sonic Adventure 2, on the Last Story, every villain and hero teamed together to get to the cannon to stop it from blowing Earth to smithereens. Except Shadow. Amy finds him in a room where he's just staring at the Earth, not even wanting to help. She eventually convinces him (along with a memory of his long dead and possibly only best/sisterly loved friend Maria), and he goes out to help just because of Maria. While everyone had a level designed to their characteristics, Shadow doesn't get anything to do until the end, when Knuckles and Sonic are ready to shut down the cannon by using the Master Emerald to put the other emeralds in control to stop the laser. The thing in their way? A gigantic, prototype lizard. Shadow finally shows up, and, in what's considered a Crowning Moment Of Awesome, starts a boss fight. Shadow is the 6th person on the team. Sonic, Knuckles, Tails, Eggman, Rouge, and finally Shadow at the end.
Originally Knuckles started out as Sonic's enemy only to eventually team up with him and Tails to form a Power Trio.
Most veteran players in Pokémon will have one 'mon in their six slot party learn nothing but Hidden Machine moves, which are needed to get through various environmental obstacles. The reason why these moves are given to one Pokémon is because they cannot be replaced by newly learned moves, and typically range from mediocre to useless in battles. As a result, trainers will end up with a Five Mon Band for matches and a utility knife who will never see the next level-up. In such cases, the player will usually pull out and raise a brand new replacement 'mon for the HM slave (usually a late-game catch, such as a plot Legendary or the Red Gyarados) so as to have a full team for the Elite Four and Champion. This is an interesting case, as it was the player, not the plot, that added the Sixth Ranger to the team.
Vandal Hearts has what feels like several of these, as the party eventually numbers out at thirteen members. Ash, Clint, Diego, Huxley, Eleni, Zoha'ar, Kira, Grog and Dolan all get their own plotlines, if not development. On the other hand, Sara and Amon just join with Dolan and contribute nothing to the plot though most egregious is Darius, who joins a little before the half way mark because he was in the same cell, tags along with the party for a reason that's never even given and only has one him-centric scene after he joins where he's revealed to have been from Huxley's village and and doesn't like him. After that he only gets the odd line that anyone could give.
Later on in Dragon Age: Origins it's possible to recruit Loghain into your group. However in doing so your Lancer Alistair will leave you for allowing the man who let countless people, including the King and his father figure Duncan to die into the Grey Warden. Few people get along with him, particularly Wynne, who insists on calling him out for his former actions whenever possible.
Dragon Age II has DLC character Sebastian Vael. His earliest appearance is in the first act along with everyone else, but he doesn't become playable until Act II. If you download the mission in Act III of a playthrough, he's more of an Eleventh Hour Ranger.
Serena Corsair in Guardian Heroes. Although technically she joins forces with Han and his friends at the very beginning of the game, she doesn't become a playable character until the player completes the game once on Normal.
In Final Fantasy XIII you have Oerba Yun Fang, who joins the party during their encounter at Palumpolum. She first appeared in Chapter 3 when she captured Snow for being an L'Cie, but later helps Snow escape and they both catch up with Lightning and Hope. This could have been rather well done and unexpected, if Squeenix hadn't given it away within a week of first announcing the character.
Mass Effect 3 has DLC squad-mate Javik. He's not necessarily the last guy to join you, depending on when you do his mission, but everyone else in your team is a returning character except James, who's with you from the start of 3 and fits The Big Guy better anyway.
SPARTAN-B312, aka Noble Six from Halo: Reach, is the newest addition to Noble Team and completely fits the trope, up to and including surpassing the rest of the group.
Carter: Six! That AI chose you. She made the right choice.
The game has an entire character class being introduced like this: the Death Knight. They were the first new class introduced to the game since launch, start the game at level 55 with equipment that surpasses just about anything that any other character may have by this point, are former minions of the villainous Lich King, tend to act explicitly antisocial (as both non-player characters and death knight roleplayers in-character) and, in a meta example, were considered the overpowered new kid on the block before ultimately being accepted.
The Forsaken (and later even the Bloof Elf race) faction joining the Horde can be seen as this taking into account that the original Horde races share a history in previous Warcraft games and settle close together whereas the Forsaken not only reside on a different continent, they also start with the lowest reputation toward their allied races in the game.
In the final route of Duel Savior Destiny Princess Crea takes up a more prominent role, turning what was originally a harem route into one that is still technically a harem route in the end, but one that is centered around her. She even gets to follow the group to the final battle area despite having no combat ability and the scenes not really being written to include her.
El Goonish Shive started with a Comic Trio and now has an eight-person main cast. Justin and Nanase aren't really Sixth Rangers, but Grace qualifies if Layla (below) does. Susan and Ellen are definitely better examples, though: Susan started out disgusted with Elliot and Tedd before befriending them through Sarah, and Ellen began her life as Elliot's self-proclaimed Evil Twin. There's also Catalina added as a Sixth Ranger. She's a buddy of Susan's from the feminist club whose main purpose was to have an unrequited love for Susan.
Aylee starts off as your standard Alien parody trying to eat our heroes, but after accidentally returning to their dimension, makes peace with them and becomes Torg's secretary, and eventually a full-fledged True Companion.
Riff's girlfriend Sasha became one of these shortly after being introduced. She was a versatile character, able to get along with the Mad Scientist Riff just as well as the Only Sane Woman Zoë. However, she stuck around after Zoë moved away, but left at the same time Zoë returned. Sasha returned seven years later, to round out the main cast after a couple vacancies opened up.
Aaron/Jackson from Sam and Fuzzy. Although most characters besides the titular duo are confined to one particular story arc (outside of cameos), Aaron managed to join with them at the end of the third story arc and has stayed a major character since.
Alexander from The Dreamer counts, as he first appears in issue #8.
Earthsong: Willow is an interesting version of this: the story starts around her, so we don't see much of the Five-Man Band before she showed up, but part of the background is that it has been there a good, long time.
Pella from Looking for Group joined the party during their time-travel adventure. Adding the Dwarven Bard was part of the writers' initiative to distance themselves from the World of Warcraft parody they started as.
Bayn from True Villains joins the group after dealing with Xaneth and surpassing the Big Bad in terms of power. Xaneth turns him into a kid to take away his powerful magic.
Shining Armor helps out his sister and her friends during Act I of Three Apples. While temporary, it underscores Applejack's growing insecurity, as he effectively replaces her as The Big Guy during their travels together.
Most team members of That Guy with the Glasses qualify, if you consider the main "team" to be the Nostalgia Critic, Linkara, the Nostalgia Chick, Suede and Marzgurl. Spoony could even be said to follow this Trope full circle- he became immensely popular and then left (Due to business deals in 2012- Spoony still appears in some videos on TGWTG).
The titular guild from Noob gets a few additions over the course of the series: Ivy and Couette joined at the same time, giving the guild a Gadgeteer Genius and a second healer. Ystos eventually joined via a second character, but was almost one beforehand between his somewhat Big Brother Mentor status and his takeovers of Sparadrap's avatar.(Those who think Golgotha is missing from the list please check Honorary True Companion.)
Season 1 started with a core cast of 5 engines(although James was only named a few episodes into the season). About halfway through, Percy was introduced. Poetically, like the Trope Namer pictured above, he's both green, and the 6th member of the team.
This also applied to Duncan and Rusty from the narrow gauge railway. Also, Emily is considered a sixth ranger to the main cast, from season 8 onwards.
Aisha of the Winx Club (introduced in the 2nd season premiere), although she's on the good fairies' side from the start, and the biggest misgiving anyone has is Layla feeling left out when Stella mentions "the five of us" in one episode. Helia joins the Specialists in season 2, while Nabu joins them at the end of season 3. Roxy is notable in that she seemed to be set up as one in season 4 since she was the focus of that season, but come season 5 and she's been Demoted to Extra.
Zuko joins the group as the more archetypal Sixth Ranger shortly after his Heel-Face Turn, with much suspicion and distrust and at least one death threat from Katara, though he quickly manages to earn their trust.
The gang invited William to join the team in "Double Trouble", only for him to get turned evil half-way into the next episode, "Final Round", and not be freed until the end of Season 4, by which time the Warriors were about to destroy XANA for good.
As of Season 5, Code Lyoko: Evolution, where it is revealed that XANA barely managed to survive, William plays the trope straight, albeit after a struggle to earn the position back.
Season 5 also introduces Laura, a Techno Wizard at least as skilled as Jeremie and Aelita, who forces her way into the team after sabotaging their Reset Button so that her Laser-Guided Amnesia wouldn't be complete. She doesn't appear regularly, but proves helpful on occasion. Aelita, however, doesn't take it kindly to have a second "Mrs. Einstein" in the picture, and the conflict between those two never goes away. It finally ends with Laura turning into a Sixth Ranger Traitor, but it only lasts for the duration of a single episode, after which her memory is permanently erased of all things related to Lyoko.
Notably, after the episode where he almost becomes a Sixth Ranger, he becomes an outright parody Sixth Ranger tropes. He dresses in a ridiculous cape and fedora, attempts to make dramatic entries, tries to be mysterious (when everybody is well aware he's the Annoying Younger Sibling of The Smart Guy) and refers to himself as "The Tommy."
Terra in the animated version of Teen Titans(until it was revealed she was a spy for Slade).
Danny Phantom: The Power Trio group is composed of Danny, Sam, and Tucker, then upgrades to include a fourth member in their Team Phantom with Jazz a little ways into Season 2, though she played Secret Secret Keeper for almost the entire series before that.
X-Men: Evolution: Rogue pulls a Heel-Face Turn and joins the X-Men after she realizes Mystique tricked her into joining the Brotherhood and thinking the X-Men wanted to kill her.
Blinky Bill - Shifty Dingo in the second season after the dingo family performed their Heel-Face Turn. He also qualifies as The Smart Guy of the team, probably due to being the weakest and most timid of the males.
Transformers Animated has two sixth rangers. The first and more conventional one is Jazz, who joins at the end of the episode before the finale, and the second is Sari, who is upgraded from Tagalong Kid. Gets weapons and armor installed and everything.
Smokescreen, Wheeljack, and Ultra Magnus all fill this role in different ways in Transformers Prime.
In the first few episodes of 6teen, Caitlin tries to earn her way into the group, but later fits right in.
In the dinosaur-flavoured Teenage Mutant Ninja Turtles-style cartoon Extreme Dinosaurs, the team originally consists of four dinosaurs - T-Bone the t-rex, Spike the triceratops, Bullzeye the pterodactyl, and Stegz the...yeah. In the later half of the series, they are sent to an alternate dimension and bring back the ankylosaur Hardrock to become their fifth ranger. Except he's barely mentioned or acknowledged even after then. Hmmm.
The 1980s Jonny Quest added two extra members to the initial five character ensemble; Hardrock, an ancient stone man who became the team's second bodyguard; and Jesse Bradshaw, a Distaff Counterpart to Jonny. Hardrock joined the group during an early episode and was featured prominently for more than half of the series, but Jessie only appeared in one episode before the series was canceled. Jessie would later appear as a regular character in future incarnations of the franchise, where she would be retconned into "Jessie Bannon" (Race's daughter), but Hardrock never returned following the 80s version.
Drawn Together has two subversions: Sid from "Drawn Together Babies" and Excludie from "Lost In Parking Space" (the first part) are both presented as ninth housemates (and Remember the New Guy characters at that), but Sid sleeps on his stomach and got sudden infant death syndrome, and Excludie was excluded from everything, even the episodes prior to his only appearance. 'Steve from Long Island' acts like this as well, only appearing in two episodes seemingly only to balance out what ever bizzare faction split the contestants had created at the time.
South Park: Butters. He's been around as a background character since the show's beginning, and gradually increased in prominence, but ascended to protagonist status during Season Six. Unlike other instances of this trope, all five leads are rarely seen together since the boys are not a genuine Five-Man Band, but Butters has had almost as many character-centric A-Plots as Stan, and Kyle by this point in the series, and will oftentimes pair off with one of the boys for an adventure ("My Future Self 'n Me," "Super Fun Time," "Sexual Healing," "Going Native", etc.) as their foil.
The boys even had an audition to see who would be Kenny's replacement after he was Killed Off for Real; Tweak was the initial winner but the writers found Butters' innocence an endearing foil to the rougher boys, and that he had more potential for Character Development.
Galaxy Rangers: Though he was technically an Ambassador, Zozo was in a surprising amount of episodes, often acting as an auxiliary, non-powered Ranger. He was even addressed as "Ranger Zozo" several times, implying he was at least an honorary, if not full, Ranger in his own right. His fellow ambassador, Waldo, was in fewer episodes, and more a Non-Action Guy - but was still capable of self defense. The show tried to make Buzzwang into a "fifth Ranger," but he wound up being The Scrappy instead.
In Futurama Kif Kroker is the team's Sixth Ranger, having come lately to the main Planet Express ensemble by means of dating The Chick, Amy. Later, Cubert also fills the role. As does Scruffy, the janitor.
The Land Before Time TV Series had two, Chomper, the baby Sharptooth from the second film, and Ruby, a 'Fast Runner' who was intrusted with Chomper's safety by his parents. Both came to live in the valley to escape the series Big Bad Red Claw.
In the Equestria Girls movie, Twilight has Sunset Shimmer join her group of human friends after she returns home. Bonus points in that she is actually the sixth to join.
Happened in universe on the AnimaniacsPinky Andthe Brain segment "Puppet Rulers", when on the Treckle and Meaney show, two new characters called "Big Ears" (Pinky) and "Noodle Noggin" (Brain) joined the show.
Scooby-Doo! Mystery Incorporated jumps around with this. When Daphne temporarily leaves the group, Hot Dog Water/Marcie acts as this until she's kicked out when Daphne returns. For a little while, Nova sticks around with the group, but this also doesn't last.
Normally, the Phineas and Ferb gang consists of the title characters, Isabella, Buford, and Baljeet. Irving is introduced in the second season as the president of the Phineas and Ferb fan club and is sometimes accepted as the sixth ranger.
The series based on Dora's Explorer Girls, Dora and Friends: Into The City, includes a new male character named Pablo.
The Iroquois Confederacy (originally the Mohawk, Seneca, Oneida, Onondaga, and Cayuga nations) became known as the "Six Nations" after the Tuscarora joined.
Sioux dialects: There are four. In Teton, in some places one uses an L, hence Lakota. However, in Santee and Yankton, one uses a D. While the N-using Assiniboine were the enemies of the Sioux, making for one more, the true sixth, well, fifth ranger, is Stoney, much farther to the northwest, which also uses an N. This wouldn't be so bad, except that their word for "ally" is Lakota. No version uses an R, though.
The United States has a law, though not a constitutional one, regarding how states join, so technically, 37 states qualify. Good for America, since Thirteen Is Unlucky. Others regard Alaska and Hawaii as this, for not being contiguous with the United States. Still others would say Washington, DC and Puerto Rico (they are not states - the former is a district and the later is an unincorporated territory - but they are frequently included in statistical reports released by the federal government and treated as though they were).
The Five Nations rugby union tournament later added a sixth team, Italy, to the existing five of England, Scotland, Wales, Ireland and France. It was renamed the Six Nations. France is actually a Sixth Ranger too, as the tournament used to be called the Home Nations, a name referring to the four countries of the United Kingdom.
Penn State became an Eleventh Ranger to the Big Ten; they redesigned their logo to hide an "11" in there without having to change their name. When Nebraska joined, the logo changed (no more hidden numbers) but the name didn't.
Whenever you have a functional team of your coworkers and a new person is hired without anyone leaving, they become this.
Many examples in international politics and warfare. A fairly famous example being the United States in both World War I (war began in 1914, US joined in 1917) and World War II (Britain and France declaring war on Germany in 1939, the US joined at the end of 1941), leading to a bit of a joke between Europeans and Americans; the former insist that the US is always Late to the Party, while the latter complain that they're the ones who have to do all the work. It's all in good fun, though (usually). The Soviet Union can be considered a Sixth Ranger in the latter as well, due to them also joining in '41 (earlier than the U.S. by way of Mistreatment-Induced Betrayalnote The Soviets partitioned off Eastern Europe with the Nazis in the early years, whereas FDR was sympathetic to the Brits but didn't have a casus belli until Japan attacked). Unfortunately, they're barred from the jokenote Despite very heavy casualties against Germany, more than the Western Powers Combined in that front, though they sat out of the Pacific War until the Eleventh Hour due to disputes about the spoils and... other matters.
In Major League Baseball after the All Star Break teams with no hope of making the playoffs will often trade their most expensive talent to teams that are still in contention. Money Ball popularized the term "Rent A Star", but they serve the same role as Sixth Rangers.
New York City has five boroughs. New Jersey sometimes gets called the Sixth Borough, given how much of it falls within its sphere of influence. Occasionally, the moniker is limited to just Hudson County, which is right across the Hudson River from Lower Manhattan.
In Australian Rules Football, the Victorian Football League began in 1897 with eight teams (Carlton, Collingwood, Essendon, Fitzroy, Geelong, Melbourne, South Melbourne and St Kilda). The Australian Football League (which it evolved into) currently has eighteen:
Richmond and University joined in 1908 (although University only lasted until 1915)
Footscray (now the Western Bulldogs), Hawthorn and North Melbourne joined in 1925, and the league remained at 12 teams for over 50 years (with South Melbourne relocating to Sydney in 1982)
Brisbane and the West Coast Eagles joined in 1987
Adelaide joined in 1991
Fremantle joined in 1995
Port Adelaide joined in 1997
Gold Coast joined in 2011
Greater Western Sydney joined in 2012
Expansion teams in closed-shop sports leagues (i.e., those in North America that don't practice promotion and relegation), whether granted and created from scratch or imported from the merging of a rival league.
Basketball subverts this with the "Sixth Man", which is the title of a team's primary substitute.note Basketball formations have 5 positions. Such subs are on the court just about as much as any of the Starters, and so the chemistry between the Sixth Man and the subs is just as important as the chemistry between the starters themselves.