Older Than Feudalism: The Bible has many examples of true companions scattered throughout (Bash Brothers David and Jonathan get the most screen time, in 1 Samuel). The more-recent New Testament refers to Jesus as a "Friend that sticks closer than a brother" which, given that in those days relatives were true companions by default, is pretty darn close indeed.
The rabbits in Watership Down, by way of all they had been through together, make one of the most endearing true-companion groups ever seen.
The Animorphs started out like this. Jake and Marco, and Cassie and Rachel, were best friends before all the crazy stuff went down (Jake and Rachel were cousins, but not close). Eventually, thanks to Character Development and saving each others skins' from danger, they become as close as family and Jake even comments so.
Since one of the most common recurring themes in the series is that war eventually ruins everything, these relationships get a lot darker and more twisted as the series go on, but they still remain fiercely loyal to one another.
The Animorphs series also gives us the Andalite term "shorm" — meaning "tail blade" in the Andalite language, it is used to refer to a friend you trust enough to let them hold their tail blade against your throat (a situation where the least twitch would be enough to decapitate you). Ax and Tobias are like this even before Tobias is revealed to be Elfangor's son—and thus Ax's nephew.
It is most obviously shown at the end of the series, when the kids see rescuing Ax from The One as a moral obligation.
Marco: "He told me the whole story. And every word was another nail in my coffin, because what was I going to do, refuse to save Ax? How many times had I been down for the count and only that lightning blue tail had brought me out alive?"
The Lords of Discipline by Pat Conroy has an unusual (read "tenuous") example of this. It's used as a plot point when the main character attempts to use it as a [semi] legal defense of his friend .
In Child of the Hive by Jessica Meats, Will, Alex and Ben fit with this trope. It's particularly evident when Ben promises to stand by Will even before Will tells them what's going on.
In the Daughter, Servant, and Mistress of the Empire books by Raymond E. Feist and Janny Wurts, the society is supposed to be much like feudal Japan. One nation divided by Noble "Houses". When you become a member of a house, you swear on the House natami (a rock) as a member. From that point on, your life is your oath and the House will protect you (even if that means rescuing you from an enemy to execute you themselves for your crime). The worst thing that can happen to someone is to be without a House.
Stephen King created his own word for this: ka-tet, which in the High Speech of The Dark Tower means a group bound together by fate. The perhaps "truest" depiction of true companions in King's world, and also the most tragic, has got to be the Losers Club from IT.
Garth Nix's The Seventh Tower: This describes the relationship between Tal and Milla pretty nicely. For most of the series, they do not like each other. At all. In fact, Milla's reaction to another character admitting he had attempted to kill Tal was "You should practice harder." Even at the end of the series the two couldn't really be called friends. But they trust each other, and know they can depend on each other in a crisis.
In Andre Norton's Catseye, Troy's ability to communicate with the enhanced animals leads to a true companions group of which he is the only human member.
Norton was very fond of this trope, particularly in her Solar Queen series. Free Traders collectively regard themselves as true companions with respect to the larger trading corporations (although they freely and very roughly compete with one another). The crew of the titular ship forms a very tight bond amongst themselves throughout the stories.
Harry seems to think like this about most people who aren't Death Eaters, especially in the later books; although it could be argued that Harry, Ron, and Hermione are a sort of inner true-companion group within the bigger group. The end of Harry Potter and the Half-Blood Prince also makes very clear the difference between the relationship between true companions and romantic relationships: Hermione and Ron are allowed to join Harry in his quest for the Horcruxes, while Ginny is not, because they are Harry's true companions and she is not.
Luna's mural of her friends is another example: Harry, Hermione, Ron, Neville, and Ginny. Given that Luna practically lives on a differentplanet, she doesn't even notice the minimal added difficulty of inter-House rivalry. As far as the inter-House rivalry goes, outside of Quidditch matches, Gryffindor, Ravenclaw, and Hufflepuff seem to get along all right.
Ginny, Neville and Luna rally together a group of their own during the seventh book.
The Order of the Phoenix
The Marauders are definitely true companions. This is probably best shown in the case of Sirius, who is taken in by James's parents after leaving home. Peter Pettigrew crosses the Moral Event Horizon by betraying James (and Lily) to Voldemort.
However, it's implied that the Marauders began to fall apart prior to Peter's actions. When reunited in the third book, Sirius and Lupin discuss that Sirius believed Lupin was a traitor before he was imprisoned, and that Lupin truly believed Sirius was guilty of the Potter's murders. That combined with Peter's choices suggest some serious discord between the lot of them, though what caused the relationship's deterioration is never explored. One possibility is that it goes all the way back to their fifth year when Sirius tricked Snape into seeing Lupin when he was transformed, which evidently caused a good amount of tension between the two of them as seen in memory scenes.It may also be that James Potter was the glue that held the group together and that the other three were never especially close, which would have made their relationships difficult to maintain once James and Lily were forced to go into hiding.
Les Amis d'ABC in Les Misérables: Enjolras, Grantaire, Joly, Bossuet, Marius, Courfeyrac, Combeferre, Jehan, Bahorel, and Feuilly (ignoring the varying amounts of Ho Yay between Joly/Bossuet, and, more popularly, Enjolras/Grantaire).
Kurt Vonnegut's word for it (in Cat's Cradle) was karass, which carries an important additional connotation that they are together for some fated purpose. A karass is the group of people whose actions shape each other's destinies or who share destiny-shapers, with no regard for whether you've met them or are even aware of their existence. So, for example, if an airline pilot becomes suicidal and decides to take his passengers down with him, the man whose rejection of his advances led to that despair is part of their karass.
Definitely applies to the direct associates of Gotrek & Felix (as well as the title pair themselves) in the Slayer series. The pair have gained and lost more than a few members of their ever-changing crew due to the grim, dangerous nature of the Warhammer world, but their enemies pay dearly for every one. Very dearly. Both of the eponymous characters have gone into unstoppable killing rages because of a lost friend, slayer or no. Though originally brought together by a binding oath to record Gotrek's mighty doom in battle to redeem himself of a past shame(The goal of all members of the Slayer cult,) It becomes obvious that Felix sticks around for more than just the Oath.
Elfslayer: Felix runs up on an exploding, magic powered submarine to pull off Gotrek, not even knowing if he's alive and in the midst of a desparate escapes sees he is breathing and thinks "Well, that's all right then."
Doc Savage and his five aides are consistently described as being "closer than brothers".
The Flock in Maximum Ride is composed of six avian-human hybrids who are completely unrelated (except for Angel and the Gasman, who are brother and sister) but have formed their own sort of "family" because they have been living together practically since birth. However, it's revealed in book 3 who Max's mother and father are, but she leaves them behind (with promises to visit frequently) because she and her Flock have to go save the world. However, this makes Max and Fang's relationship more than a little awkward. Max also states more than once in the books that the Flock are the only five people on the planet whom she trusts implicitly, and Fang seems to have his own even higher level of trustworthiness within that circle.
Republic Commando: The key concept in Karen Traviss's take on the Mandalorians in her novels. This is lampshaded in the fact the Mandalorian language doesn't have separate words for "friend" and "family member," and has a proverb that translates to "Family is more than blood." At least two Knights of the Old Republic fanfics have Canderous "adopting" Revan and the rest of the crew.
Carth: From what I do know of Mandalorians, ones without a clan have no place in their society, but they can offer themselves to another clan in hopes of being... well, of being adopted. Mission: So did we just adopt Canderous, or did he just adopt us? Carth: I think the answer is "yes," Mission.
Following on from the other D&D-based examples, the Heroes of the Lance in Dragonlance.
In his alternate history novel The Years of Rice and Salt, Kim Stanley Robinson uses the device of the karmic jati, or a group of individuals who are reincarnated in close proximity to each other throughout successive lives, to maintain a link between his characters over the seven centuries of narrative. The same half-dozen souls reincarnate into each stage of the story, maintaining their individual personalities even as their names, genders, ethnic origins, social standings and species change.
Guy Gavriel Kay's The Lions of Al-Rassan deals with an unlikely crew made up of two Crystal Dragon Christians, two Crystal Dragon Muslims, and two Crystal Dragon Jews. They repeatedly do things like ride hundreds of miles to protect the family of other true companions, or fight off members of their own ethnic group to protect their crew.
Vikram Seth's novel An Equal Music is largely about the bond between four musicians performing as a string quartet.
The Wheel of Time has several examples. The Aes Sedai, the Aiel clans(particularly the Maidens Of The Spear), the first main group from the first book. Etc, etc, etc.
The four protagonists of Circle of Magic and its sequels eventually begin to refer to themselves as siblings. What can we say; living together, training together, fighting off all manner of dangers (from earth-quake to plague) and having your very minds linked together tends to do this.
Tamora Pierce's Tortall books almost always feature a protagonist with some kind of super-power - broadly defined. Mostly this is magical. But Keladry of Mindelan's superpower is actually the ability to create true-companions relationships among the people around her.
In Robert A. Heinlein's Stranger in a Strange Land, the main character (who was raised on Mars) thinks sharing water with someone means forming an eternal bond of love and trust. Because he's very special, the first half dozen or so people he "shares water" with really do feel connected through him, so they quickly end up forming a close-knit group. After that, more and more people get invited to share water with the group, eventually forming the beginnings of a worldwide cult.
The Dresden Files: Harry has a talent of sorts for this; not only has he forged bonds with Murphy, Thomas and several other wizards, he's been true companion to (at least) three Knights of the Cross, the Archive, and a broad selection of pixies and other lesser Fay (who granted him the title of 'Za Lord'), complete with a loyal (and adorable) guard. Plus Mouse.
"I don’t care about whose DNA has recombined with whose. When everything goes to hell, the people who stand by you without flinching—they are your family."
A talent? Let me rephrase: Harry Dresden is the king of this trope. When a Black Court Vampire threatened one of Harry's friends, he told her that he'd destroy the world just to unmake her if she tried. When he calls for help, gods answer. Forces of Nature rally to his call. He has a personal nickname for Uriel. His friends have followed him to the heart of Arctis Tor, home of Mab, to the heart of Chichen Itza, home of the Red Court, to the lair of a Wizard-level Black Court Vampire, to the home of the White Court, into a war amongst the Fae, into a necromantic battlefield, and more. It's hinted that he's going to surpass this in the future, routing the Outsiders at the forefront of some reasonable fraction of humanity.
And if Harry Dresden is the king of this trope, Tavi Calderon of Codex Alera is its patron saint. He gathers true companions like we mortals breathe. He turned hostile barbarians with a mutual grudge into close allies, their angry princess into his fierce and loving wife, an army of telepathic yetis with a millennial grudge into neighbors, conquering wolf-warriors into the closest of frenemies, political enemies into vassals, traitors into lieutenants, and even tried it with the Vord Queen. In fact, the only parts of his plans that are remotely foreseeable are this trope and him being in the craziest possible place for him to be.
The members of The Raven in James Barclay's Chronicles of the Raven and Legends of the Raven novels. They never actually swear anything beyond their normal Badass Creed, but they consider the Raven to be more than family, in some cases abandoning their real families. And, together, if they're on their game, however many of them there are, no matter the odds, they will win, or make a very good attempt.
Burke's "family" in Andrew Vachss's Burke novels (Flood through Another Life) are all criminals who were failed by the system and their biological families at early ages, but have chosen each other, and stick as close as any family.
Gaunt's Ghosts: Gaunt and those who went on the Gereon mission with him.
Gaunt: "I still take a pause to think that Rawne's a friend of mine now. My best friend, to be honest. Time was, we'd have happily killed one another. I still hate him and he still hates me, but the necessity of Geron bound us tight. Feygor too. Not a model soldier, but after Gereon, I'd have sold my soul for him and vice versa."
Soraya and her half-brother Jiaan in the Farsala Trilogy, eventually. They start out strongly disliking each other, but eventually put aside their differences. It helps that Soraya realizes Jiaan is the closest thing to family she has left.
A funny example of this in Ender's Game and its sequels, specifically the "Shadow" books. Ender's "jeesh" become much closer then they are with the families they hardly remember, united under Ender's leadership. The only exception being Ender himself who is cut off from the others.
There are several the Shadowleague books by Maggie Furey, but the main one is between Elion, Kazairl, Toulac, Zavahl, and Veldan.
Monstrous Regiment: The members of the regiment come together to protect each other and their identities until they have to reveal themselves to Sgt Jackram in order to complete their individual quests. Even after capture, they refuse to leave a comrade behind.
The Ankh-Morpork Watch. At first, Colon and Nobby are all the family that Vimes has. By the end of Guards! Guards!Carrot is adopted as one of them. As the Watch expands, there is heavy emphasis on new recruits realizing the importance of relying on and protecting fellow officers.
Vimes: We're just one big family and, when you've been to a few domestic disputes, Littlebottom, I can assure you that you'll see the resemblance.
Going Postal: When Moist von Lipwig arrives at the Post Office it already houses a mini-crew in the shape of Groat, Stanley and Mr. Tiddles. He and Mr. Pump join this little family.
Arguably, the wizards of the Unseen University: They may not like each other, especially at meal times when they're fighting for the last goose, but ever since Ridcully put a stop to the old way of doing things, they've made it very clear that only wizards are allowed to disrespect wizards.
Similar to the above, the witches, especially Nanny, Granny, and Magrat/Tiffany/Agnes. Getting witches together is like trying to herd cats, but they visit each other regularly to make sure they're all sane, and will band together if the threat is great enough.
Great Alta Saga: Jenna's band. Though it eventually develops into an army, for most of the book Jenna has few enough followers that she can afford to be very loyal to all of them. In general, all of the Hames were designed to be this way.
On the Jellicoe Road: There are two, tied by blood. The first is Narnie, Tate, Fitz, Webb and Jude, also known as Hannah, Taylor's mom, the Hermit, Taylor's dad and the Brigadier. The second one is Taylor, Jonah, Raffy and Chaz.
Chaz: What are you so sad about? We're going to know him for the rest of our lives.
From Malevil, the survivors in the eponymous castle. There is a saying in their region: "Each will strengthen each", which they take to heart and truly live for each other, from the members of Emmanuel's childhood "Club" to the newcomers from La Roque and L'Étang. Surviving the Apocalypse is merely survival, surviving the Apocalypse with their large extended family is actual living.
Dracula: Jonathan and Mina Harker, Van Helsing, Dr. Seward, Lord Godalming, and Quincey Morris form one when they vow to slay the vampire that killed Lucy. Later, after Mina gets an evil scar following her own encounter with the Count, as Jonathan embraces her in her anguish, their friends silently join them:
"Then without a word we all knelt down together, and all holding hands, swore to be true to each other."
The Crew in Stuck are this already when Tre meets them all in the mall, but the events of Episode One eventually lead to them letting him in, and soon enough they become truly like a family rather than just being friends.
In the David Drake RCN series, Daniel Leary realizes that Adele Mundy, Hogg, and Tovera and himself have become an odd little family.
The Heraldic Circle in the Heralds of Valdemar series. While individual Heralds may have issues with other Heralds, as a whole they are all close and stick together through thick and thin.
Twenty Thousand Leagues Under the Sea: Captain Nemo claims (and the few interactions Aronnax had with the Nautilus crew never shows us any different) that this is the relationship between the crew:
.."The Nautilus suffered a collision that cracked one of the engine levers, and it struck this man. My chief officer was standing beside him. This man leaped forward to intercept the blow. A brother lays down his life for his brother, a friend for his friend, what could be simpler? That's the law for everyone on board the Nautilus.
Henry Wadsworth Longfellow gives us this example in the Song of Hiawatha:
"Two good friends had Hiawatha,
Singled out from all the others,
Bound to him in closest union
And to whom he gave the right hand
Of his heart, in joy and sorrow ;
Chibiabos, the musician,
And the very strong man, Kwasind.
In Death series: Eve Dallas ends up getting this. It consists of her husband Roarke, her best friend Mavis, her father figure Feeney, her mother figure Dr. Mira, and her friend and partner Peabody at least. Roarke, on his part, has Summerset for a father figure, and the people listed for Eve.
Sisterhood Series by Fern Michaels: At the beginning, we have Nikki Quinn, Kathryn Lucas, Julia Webster, Myra Rutledge, Charles Martin, Alexis Thorne, and Yoko Akia. Then Julia dies, and Countess Anne "Annie" Ryland de Silva takes her place. Lizzie Fox and Maggie Spritzer become part of it. Then a group consisting of Jack Emery, Harry Wong, Bert Navarro, Ted Robinson, and Joe Espinosa eventually gets formed. Yes, it gets a little complicated after awhile.
Onyesonwu, Luyu, Diti, and Binta become True Companions in Who Fears Death thanks to all undergoing the Eleventh Year Rite together. Later on, they, along with Mwita and Diti's husband Fanasi decide to accompany Onyesonwu on her quest West.
'The Floating Outfit' in the novels of J.T. Edson.
Women of A Brother's Price have the most power when they are formed into families, bands of sisters. Women of lower classes who weren't born to families, or were disowned or left, can band together and legally be considered sisters even if they are completely unrelated. They have to have a clear sense of who leads, though, and not have conflicting interests. The Whistler family started with one such band.
Katniss, Haymitch and Peeta in The Hunger Games trilogy. This goes for all the victors really, since it is shown that the group has some very close knit friendships between them. The other victors hold the distinction of being just about the only people Haymitch considers his friends, which considering the kind of guy Haymitch is, is quite an accomplishment. And Katniss and Peeta, over the course of the books, form close friendships with fellow victors Johanna Mason, Finnick Odair, and Annie Cresta, among others. This is quite understandable, as when people go through (as adolescents) something as incredibly traumatic as the games, it's only natural that they would become very attached to the around forty strong group of people who have gone through the same thing, not to mention that the victors are just about the closest thing Panem has to celebrities. The victors are not friends because most people don't understand, they're friends because nobody else understands.
The group from Ljuder Parish in "The Emigrants". In Unto a Good Land the text outright states that they have set aside their past differences and become one household and one family.
In Nnedi Okorafor's Akata Witch, the four main kids come together to form an Oha coven which is said to bear the responsibility of the world on its shoulder at a specific moment of time.
Legacy of the Dragokin: The comradery between the evil Kthonian Knights is their only redeeming trait. This is die-then-come-back-to-life loyalty.
In The Heroes of Olympus it takes the Seven a while to grow into this, but by the end of The House of Hades even the most reluctant of them are incredibly close and protective of each other.