Nakama, the original term for True Companions, came from anime and manga; and to no one's surprise, you'll find a ton of examples here.
Any character that Kenshiro befriends in Fist of the North Star becomes his true companion not long after. The Power Trio of Kenshiro, Rei and Mamiya is probably the most famous example, but Toki, Fudoh, Shu, Ein, Falco and of course Bat and Lin are also as undyingly loyal to him as he is to them.
Monkey D. Luffy's crewmates. Especially evident in the Enies Lobby story arc, when the crew declares war on the World Government (a ruling body that encompasses over a 170 different countries) in the name of crewmate Nico Robin. The statement they made was that they would never betray her over her "burden". Instead, they chose to make her burden their burden.
In the Whitebeard's War arc, a full-scale war between pirates and the government erupts just to save Ace. Also notable because Ace is revealed not to be blood related to Luffy, instead being Gold Roger's son.
As Whitebeard's life flashed before his eyes, we see a younger Newgate express that this is all he really wanted. He succeeded.
Early on, the crew pledges to fight Arlong for Nami's freedom, despite the fact that she had betrayed them and taken off with their ship and all their treasure.
At the end of the Water 7 adventure, it seemed to be a test of whether Usopp understood what it meant to be true companions.
At Thriller Bark Zoro is prepared to give his life to save Luffy's. Sanji also attempts to sacrifice himself but is stopped by Zoro who in this way effectively saves two of his true companions at the same time.
Speaking of Thriller Bark, this trope is played a little darker in the case of Gecko Moria. It is heavily implied that his former crew was much likeLuffy's... until they were all killed by Kaidou, marking his Start of Darkness.
The core teenaged cast of Ranma ½ can possibly be interpreted as true companions, particularly in the later seasons and OVAs. This still didn't keep them from beating each other up at the drop of a hat. A dramatic shift in the Ranma-Ryōga-Mousse dynamic took place during the Musk Dynasty arc. Since Ranma is locked as a girl, Ryōga and Mousse plot to let Herb kill her (or do so themselves) while they remove their own animal curses. However, when they start to put their plan to action, and it backfires, Ranma doesn't just fail to begrudge them, she actually yells at them to save themselves even as Herb is about to kill her. Later, Ryōga and Mousse come back to help Ranma unlock the curse and save her life from Herb. Notably, and despite the unavoidable bickering and squabbling later in the series, the "bitter-rivals" relationship changed to "loyal-allies" after this event.
The cast of Tenchi Muyo! is another such example, although being that they are all related, if distantly and via fusion in places, the label for this is somewhat murky.
The Sailor Senshi. It's only fitting, consider how many suffer from Parental Abandonment. This is lampshaded in the Image Song for Sailor Mars and Mercury called "Hottokenai Yo" (Loosely translated as "I Can't Leave Her Alone!"):
I can't leave her alone, because she's like family, I'll combine forces with her.
This was also one of the first things we learned about Sailor Uranus and Neptune's relationship to show they weren't just Sixth Rangers.
The Shitennou of the Dark Kingdom fit the example in the original manga. Kunzite, Zoisite, Nephrite, and Jadeite were like brothers and were visibly upset when members of the group were killed. In the anime, however, they were happy to backstab one another, except lovers Kunzite and Zoisite.
Ronin Warriors: the leads are True Companions. For the majortiy of them the Ronin are their family for all intents and purposes as their actual parents are either dead, or are physically or emotionally distant. This is largely subverted with the four Dark Warlords, who for most of the series don't give a damn about each other.
In Katekyo Hitman Reborn!, Tsuna, Gokudera, Yamamoto, Ryohei, Hibari, Mukuro, Chrome and Lambo are true companions in a more literal sense of the term.
Star Driver: The Midnight Flight Drama Club were already true companions, but the arrival of Takuto seems to be bringing them closer and closer together.
The good guys in Dragon Ball Z are definitely true companions: they're all close friends, despite going anywhere from three to ten years without speaking to each other, and although Vegeta spends the rest of his life resenting how Goku beat him up, he eventually becomes part of the group.
This actually goes further back to the original Dragon Ball. It started out with just Goku and Bulma, then we have a bandit and his sidekick, and a pig who won't "behave" himself.
Furthermore, the original Chinese novel that inspired it, Journey to the West (which is where the "real" Son Goku comes from), is also an example.
The Ginyu Force could also count as True Companions, even more so between Burter and Jeice.
The core heroes of all six Digimon series. Although they don't always get along initially, they usually become true companions fairly early in the plot. Former villains or new characters may become true companions in the second half of the show.
The Bronze Saints from Saint Seiya, very much so. The Gold Saints to an extent too.
Every one of the sagas in JoJo's Bizarre Adventure depicts a protagonist group of true companions, but probably the third's (Jotaro Kujo, Joseph Joestar, Noriaki Kakyoin, Jean-Pierre Polnareff, Muhammed Avdol and Iggy) are the most close knit of them all. In fact, the ending of that saga is actually a Crowning Moment Of heartwarming, a considerable feat for such a violent, macho series.
After an amnesiac Greed kills Bido, Ling whose body Greed shares demands to know why Greed killed someone who was supposedly his friend. Greed at first argues back that Bido was only that to the 'first Greed', but Bido's death by his own hands causes Greed to regain his memories and do a Big "NO!", and subsequently attack King Bradley, wanting to know what happened to his "possessions" and why King Bradley is in his memories. What made Greed mad wasn't that Bradley killed his friends. It was that Bradleykilledhisfriends. Also, right before his Heroic Sacrifice at the end, Greed quietly admits to Ling that his greatest desire, in other words the thing he was most greedy for, was for companions. He dies acknowledging Ling and Ed as being his friends.
Ling refers to Lan Fan as "nakama" when he begs Hawkeye to allow him to pick her up from the sewers, where she is bleeding to death after severing her own arm when he refused to leave her behind.
Mustang and his five closest subordinates (plus Major Armstrong) are very much like this, in part because Mustang is A Father to His Men. Word of God says that by the time of the story, the six of them had developed a very tight familial bond after working closely together for so many years. Arguably this is particularly true of Hawkeye, who (to her knowledge) doesn't have any other living family.
To show how much Scar has developed, he refers to May Chang as this.
The Homunculi are family all coming from the same being, Father. Out of the siblings, Lust, Gluttony, and Envy are the closest, with Gluttony and Lust the closest together, as shown by Gluttony's devastation when Lust dies. This carries over into the 2003 anime version (despite the fact that Lust really didn't care about him; she just wanted To Become Human).
There is an episode of Martian Successor Nadesico dedicated to sending up the concept of true companions; it includes what is possibly one of the funniest sequences in the series.
Lottie in Soukou No Strain refers to her team as true companions and tries to convince Sara of such.
Naruto: A major theme is that the entire Leaf is this; the Will of Fire is the spiritual foundation which says the most important thing in life is being willing to risk your neck for someone else. It is not confined to the Leaf and many characters - Naruto, most notably - form such a bond with many people he meets. Most other villages have their own version of the Will of Fire too, if not its Martial Pacifist inclination.
Probably the most striking example is Shikamaru's speech to the squad retrieving Sasuke about how he personally didn't care much for Sasuke, but would try to bring him back anyways, because he was their nakama. Also a very good illustration of the nebulous nature of the term. The dub used "comrade", in the military sense of the term.
When Sasuke crosses the line on allying to Akatsuki for revenge against the village and the other nations demand his head, Shikamaru decides he's too much trouble, as having another village killing him would incite a Cycle of Revenge from Ino and Sakura, and decides to kill him. As stated above, Sasuke was only considered comrade by his peers mostly from military sense, except for Naruto. Simply put, they decided he's no longer one of them and must be taken down.
Another example later in Naruto involves the difficulties Naruto and Sakura have accepting newcomer Sai to their team; despite his betrayal, they still see Sasuke as True Companions, and Sai is treated with much hostility as an interloper. (His grating, offensive personality doesn't really help matters.)
During the "Rescue the Kazekage" arc, while describing Naruto's bond with Gaara to Chiyo, Kakashi specifically uses the word 'nakama.'
All of the teams named with numbers (Team 7, Team 8, Team 10) are groups of four people. Through the powers of Character Development, they all become tight-knit.
This is established early on by Kakashi, just after he passes Team 7 in his established test - he states in as many words that in the wolrd of ninja, although breaking the rules makes you "scum", adandoning your comrades makes you "worse than scum", and thus passes them for supporting each other. This theme only grows throughout the story.
The School Festival in Manabi Straight has "from friends to true companions" as its theme. The "friends" in this case is a different Japanese word, tomodachi, which indicates a shallower level of intimacy.
The four main characters of Hunter × Hunter are definitely this. The Genei Ryodan are also this and a rare villainous example. Shooting Star City is a mix of Breakfast Club and True Companions on a mindbogglingly massive scale. Maintain some semblance of a society among the trash heaps of the world's largest landfill takes some teamwork.
Akira, Shuji, and Nobuta of the Japanese drama Nobuta Wo Produce. Despite some romantic tension between certain members, the three generally function as true companions later in the series, and recognize themselves as such.
The five members of BECK. As Koyuki puts it in the final chapters: Ryusuke pulling him along through thick and thin, Taira always giving him the push to move forward, Chiba's cooperative spirit giving him strength and Saku always believing him. And they return the feeling in their own ways: thanks to Koyuki, Ryusuke relearned how to smile and take pleasure from his music; Taira learned to socialize with his friends; Chiba became more assertive to his surroundings; and Saku strived to become a better drummer due to the faith Koyuki had in him.
In Bleach, Ichigo and his circle of friends, including the shinigami and random defeated foes along the way.
Ishida often refuses to acknowledge the other members of the group as his friends, but goes to extremely great lengths to assist them, to the point that he once temporarily lost his supernatural powers and in several instances has nearly died.
And Rukia bitchslaps Ichigo for leaving her and Renji behind when he goes to Hueco Mundo, citing that exact same reason.
And surprisingly Starrk. The death of his true companions drove him serious. Kyoraku could be said to have similar motivations during that fight.
Chad fights for Ichigo and protects others because he was taught he shouldn't pick meaningless fights for himself, but Ichigo is mainly the reason for him doing this after they kicked ass together. Similarly, Orihime chose to join the Soul Society invasion because of her desire to protect Ichigo. Both characters also level grind during the timeskip to protect Ichigo while the latter is powerless
Squad 11 is sort of like a close knit group or dysfunctional family, especially the named members of the squad in the series, but all of them are true companions including the unnamed goons. In fact in their special closing credits image, where it's usually just the Captain and Vice Captain, the entirety of Squad 11 was there instead.
Ikkaku defines it perfectly by telling Shishigawara that, "There's no pecking order between men's lives! If you're going to risk your life, risk it equally! It makes no difference if it's for a boss or an underling, only die for those that would die for you!"
The series also sneaks in a Deconstruction that nearly breaks Ichigo: what if someone could make your friends think they were a True Companion? Well... Then it's reconstructed: the person responsible (Tsukishima) runs up against someone he can't brainwash ( post-Character Development!Byakuya owes Ichigo a debt he has to repay even if it means striking down someone he believes has been with him his entire life, and he's honor-bound enough to go through with it) and once he's defeated the Fake Memories vanish.
In Mirage of Blaze, both the Kagetora clan and the Uesugi army are eventually betrayed in favor of Haruie and Nagahide.
The Basketball Teams in Slam Dunk tend to be like this. There is also Sakuragi's gang, who might tease and laugh at his expense but still stand by his side when he truly needs their help.
The main characters of Red Garden. The girls all hang out with different cliques, and they never would've even spoken to each other if it hadn't been for Lise dying.
Both the Sanzo-ikkou and Kougaiji-ikkou in Saiyuki. While the latter are very open about it, the former will deny it until they're blue in the face. The Konzen-ikkou in Gaiden are somewhere in between: they're not as comfortable together as Kougaiji's group, but they're a much more cohesive unit than their reincarnations.
In an episode of Hell Girl, Hone-Onna asks if Ren considers her and Wanyuudo to be True Companions, and is patently dismayed when he replies that he does not. He later mutters to himself that he actually sees them as kazoku, family, which implies that this is the only thing closer than a true companion. This is especially poignant as none of them actually have ever had families. Enma Ai is also considered a True Companion as well: they'd do anything for her, and her extremely rare moments of warmth are reserved for them.
Yes, even Hiei who might not seem like this most of the time. But after watching all of Chapter Black. You can tell he truly cares for his friends.
The Lagoon Company and Hotel Moscow in Black Lagoon, both to themselves and each other. The reason why the Hotel Moscow unit under Balalaika act the way they do is because they used to be a unit of the Soviet Army, and so therefore their bond is that of soldiers. However, Balalaika's relationship to the Lagoon Company, a band of mercenaries that are not always on the same side as her, but who she is quite friendly to, is a good example of true companionship. Revy and Eda's relationship is another good example.
The five primary Blade Children in Spiral. It's worth noting that The Stoic Eyes is the one who actually uses the word "true companionship," when he tells Rio not to get herself killed.
Everyone in Get Backers forms some kind of complicated extended True Companions, with "That doesn't mean I like you" and "The second we're not being paid to work together, all bets are off" in full effect. The various members of the VOLTS hierarchy in the first Mugenjou arc are probably the best example, however. Emishi wanted to treat Shido to a good time before killing him because they were true companions once; later, after foiling his double-suicide attack, Shido drags him to the doctor. When asked why he would save an enemy, Shido smiles and answers that while they are enemies now, they used to be true companions; one day, they could be true companions again.
The five main characters in InuYasha (Inuyasha himself, Kagome, Miroku, Sango, and Shippo). When Miroku and Sango fall in love, he tells her that he doesn't want an actual relationship with her until they have completed their mission, because he values her too much as a "companion in battle".
The Ala Alba, which is Negi's true companions group. Jack Rakan even explicitly refers to them as such.
Negi's father, Nagi, had the Ala Rubra, who were his true companions group.
For the students their club organizations tend to make true companionship-like bond. The two most clearly shown are The Library Exploration Club and the Baka Rangers (due mostly to their making up the core of Negi's Ala Alba). The Sports Girls have been getting a similar treatment since they took a bigger stage the latest arc.
Likewise, the tenants of the Hinata House in Love Hina qualify. Witness the way that every time one of them leaves, the others all go out to try to find them. Even Kanako, who joins up late and is initially hated by all of the others, ends up becoming part of the "family" by the end.
Also the main characters in Ai Yori Aoshi, which at first glance seems like a Love Hina clone. (They're very different in tone, though, as well as in their emphasis on realism vs. fantasy.) Completely understandable in the case of male lead Kaoru, in that he really has no family to return to. Several of the other characters have issues with their families, too.
The Band of the Hawk in Berserk certainly qualifies as true companions, especially later on in the anime. The leader of the Godhand, Void, uses the term "nakama" when they offer their Deal with the Devil to him during the Eclipse. Griffith's acceptance of their offer and betrayal of his true companions is one of the reasons why Guts wants him dead. Guts gains another group of true companions two years after the Eclipse, with Casca being the only survivor of the previous band (though given her traumatized post-Eclipse state, she's not going to be participating in any fighting anytime soon).
If you don't hear this in Fairy Tail, you're deaf, stupid or both. Comes complete with moments of "but s/he's your nakama!", among others. Being true companions is practically the whole premise. Natsu tends to be the most vocal in terms of the True Companionship; nothing pisses him off more than a baddie who will either injure or kill their own team members, except possibly a baddie who attacks anyofhisclosestfriends.
True Companionship is even more so obvious [though not discussed/mentions as often] in Hiro Mashima's other (now less popular work Rave Master. Musica gave up the only woman who he ever could really connect with romantically — and nearly himself — for the team, and the final battle is one long string of various suicide attacks just for Haru. And that's the tip of the iceberg.
The SOS Brigade very gradually become true companions over the course of the novel series, in spite of three of them being secret agents with competing agendas and the other two being Kyon and Haruhi. Eventually it gets to the point where in one of the light novels, Itsuki tells Kyon that if his Organization ever comes into conflict with the SOS Brigade, he will betray them and side with Kyon. The same for Yuki. They were both heartwarming moments.
In the 11th book, Itsuki makes another speech declaring that the SOS Brigade is the most important thing in his life and that he, Yuki, and present-day Mikuru will always be loyal to it.
The four main heroes of Chrono Crusade are like this, possibly to replace their lost family members (they're all orphans except for Chrono, who betrayed the members of his race twice-over). This is probably the reason why Chrono and Rosette don't become the Official Couple until the very end.
In the Chrono Crusade manga, the Sinners are shown in Chrono's memories to form an odd sort of family with Mary Magdalene — which makes Chrono's eventual betrayal all the more traumatic to the rest, and also shows how ruthless Aion is when he proceeds to sacrifice them one by one for his goals.
The title group is portrayed as being like a family — to the point that Tamaki calls himself "father", Kyouya "mother" and Haruhi his "daughter". When Tamaki begins to develop romantic feelings for Haruhi, he constantly denies them by focusing on their relationship as "father and daughter" (to the point of squick levels for some fans).
Tamaki has an effect on all of them that he doesn't even realize; though they all have specific connections within their group, it's Tamaki's influence that ties them all together as one. Arguably, Haruhi also has this effect to a lesser degree, making her and Tamaki a bit of a power couple when it comes to group dynamics. There'd be shit going down if anything happened to any one of the Host Club, but if anything happened to Tamaki specifically, it'd be a full-on war with the rest of the world. And that's exactly what happens, with the Host Club calling in the entire rest of Ouran Academy as backup for good measure.
The Shuffle Alliance in Mobile Fighter G Gundam. Also, Argo and his pirate crew, which he specifically refers to as his "true companions".
The cast of Transformers Super God Masterforce forms a pair of these. Unusually, the true companions sense is more pronounced with the Decepticons, which eventually leads to Clouder, Wilder, and Bullhorn joining the Autobots after King Poseidon and Devil Z abandon them.
Allen in D.Gray-Man considers his fellow Exorcists his true companions. Even Kanda, with whom he is constantly having glaring matches when they're not fighting Akuma. Lenalee even more so. In fact, she has outright said that she doesn't honestly care about the world outside her true companions, so long as they are safe and happy she will continue to fight.
The six party members from Record of Lodoss War. The way their interpersonal dynamics go, it may be a dysfunctional one at times, but they definitely still count.
Monster Hunter Orage: The lead character. His teammates even comment on this fact, with varying levels of annoyance.
Really, Ash and all of his Pokémon party could be called this. It's part of the reason why he's a pretty good trainer, because he respects his Pokémon and they respect him.
The Pokemon that he is undoubtably the closest to are the "Gang of Four" (as they are called), Pikachu, Bulbasaur, Squirtle, and Charizard. These are four of his oldest Pokemon (Pikachu being his starter) and his relationship with them is far closer than it is with any others. While only Pikachu is currently with him, the bond the four shared with him was symbolized in a relatively recent episode where he was reunited with the other three for his third battle with the Pyramid King Brandon (and finally won).
Ash also forms a true companionship with his travelling companions, creating new bonds each time he travels to a new region. Arguably, since he's more than willing to befriend and/or help almost anyone he runs into, he's the nexus of an ever-growing super-true companionship of people and Pokémon across the known regions of the Pokémon World. Even his rivals grow to accept him as a companion.
The regional trios (and Kanto foursome, counting Yellow) in Pokémon Special fit this trope - the bond between the Sinnoh Holders, especially, is emphasized in the seventh and eighth chapters.
The Sacred Swordsmen of Unova: Terrakion,Cobalion,Virizion and Keldeo.
By the end of Baccano!, quite a few of the ImmortalLovable Rogues of the series (to wit: Firo, Maiza, Ennis, Czes, the Gandor brothers, Isaac and Miria, as well as some others) have forged one of these — a huge contrast to the original set of the highly distrustful immortals in 1711 that had be scattered just to keep them from eating each other. Nice and Jacuzzi's gang is clearly one as well, though Jacuzzi seems to treat everyone like a member of his true companions.
Jin, Mugen, and Fuu in Samurai Champloo exemplify the trope, since Jin and Mugen are in a state of indefinitely delayed duel to the death, and refuse to let it go.
The five ( or six, if you count Cloney) main characters from Tsubasa Reservoir Chronicle. Goes so far that at one point, trying to explain their dynamic to an outsider, Fai jokingly/seriously (the outsider can't tell which) says that Kurogane is Syaoran, Sakura and Mokona's father. The nicknames "Kuro-daddy" and "Fai-mommy" have since caught on in the Fandom.
Fanon portrayed the Nordic countries (Sweden, Finland, Norway, Denmark and Iceland) as this in modern-day settings, and then they were confirmed as this trope in the Hetalia Bloodbath 2010.
In Paint it, White! while the Allies (except for Canada) and the Axis powers don't like the idea of working together, they still look out for each other when they're in danger of an alien invasion. In the final battle, a few of them are assimilated because they were distracted/concerned with other members of the group being assimilated around them.
The Allies even call themselves this in an Image Song, with Russia wondering to himself if it's true.
This is also a major part of most of RPGs that have spawned from it: All the nations fighting together to escape/survive whatever is thrown at them (be it weird over-grown insects or giant alien-like monsters.) This often results in heart wrenching scenes when one puts another's life above his own and sacrifices himself 'so they can get out alive'.
Celestial Being seems to become this in the second season of Gundam 00.
The Cyber Coil Detective Agency along with Daichi and Denpa make one big group of "true companions" in Dennou Coil. There are also a few smaller circles such as Tamako, Haraken and Yasako, but the most notable is Yasako and Isako, who come to have their own special true companion relationship.
There's a few groups in Gintama that sometimes duke it out but sometimes overlap: The Yorozuya (Gintoki, Shinpachi, Kagura and Sadaharu), The Shinsengumi (Kondou, Hijikata, Okita, Yamazaki, etc.,) and the sort of disbanded Joui (Gintoki, Katsura, Takasugi and Sakamoto). However, by the end of the Benizakura Arc, it is pretty clear that whatever ties and sense of camarederie that Takasugi has had with Gintoki and Katsura in the past have been completely broken, with both Gintoki and Katsura declaring war on Takasugi for his actions.
In the manga Vagabond, the Yoshioka school of swordsmanship as a whole are essentially an independent samurai clan that, whatever their feelings about eponymous head and playboy layabout Yoshioka Seijuurou, are extremely devoted to his younger brother Denshichirou to the point that disciple Otagurou Hyousuke offers his arm (shattered in a training accident) as a "sacrifice" with which Denshichirou may proceed with his training, they accept senior disciple Ueda Ryouhei as their leader, perhaps even the "soul" of the Yoshioka all along, after Miyamoto Musashi cuts down the two brothers. Unfortunately this is the undoing of both themselves and the school, as Ueda's able to get all seventy remaining swordsmen except for the crippled Hyousuke to join in his plan to ambush and kill Musashi at Ichijouji, but the plan isn't changed even when the leaders know that Musashi's found out. On the individual, tactical and leadership levels they have not prepared to fight in what's "no longer a duel," and except for the absent Otagurou all of them pay for this failure with their lives, ending the Yoshioka school.
Twenty's gang in The Daughter of Twenty Faces is like this. Chiko even thinks of Twenty Faces as her father, and refers to him as such at least once, and calls Ken "Ken-nii-chan" (roughly, "big brother Ken"). Even after episode 6, Chiko is a true companion with Shunka and Tome.
Nanoha, Fate, and Hayate consider their relationship as this. Nanoha and Fate helped Hayate with her predicament in the Book of Darkness incident so she and her knights helped them get their positions as instructor and enforcer, which in turn, prompted the two to join Hayate in forming Riot Force 6. Hayate comments that the three of them will be stuck helping each other for life. Nanoha and Fate have no problems with this arrangement.
Hamel, Flute and the rest from Violinist of Hameln are this. It is actually stated within the manga that as most of the group had lost their mothers, Flute was like a mother to them.
The main cast of CLANNAD; what were you expecting from a show where the main theme is "family?" Tomoyo even uses the actual term when she talks about what she considers to be the most important thing in life, obviously referring to Tomoya, Nagisa, and the rest.
All four Yu-Gi-Oh! series. In fact, one could say "true companionship" is the entire point of the series (other than them dueling). And the families the characters start out with grow with impressive speed. It's one of the 5D's buzzwords, along with 'kizuna' (bonds), 'kibou' (hope), and 'zetsubou' (despair) (the last for the villains of the second half of the 5Ds. However, even they eventually rejected it in favor of hope.)
The protagonists of Project ARMS fit this. At one point, they are described by an enemy as parts of a whole body (Yugo as the "ears", Kei as the "eyes", Takashi as the "legs", and Ryo and Hayato as the "arms"). In the anime, they jokingly refer to each other as "comrades in ARMS".
Debris Section, the heroes of Planetes, qualify, especially considering they spend all day, every day together and still hang out on their (rare) days off.
The Girl Who Leapt Through Time has spades and spades of this trope. Makoto has a lengthy conversation with both her aunt and the two other members of her true companions group, Kosuke and Chiaki, about how their group dynamic would change if any of them started to date, be it outside their group or with each other. In fact, Makoto goes to incredible lengths with her time-traveling powers to prevent Chiaki from asking her out due to this.
Watanuki, Doumeki and Himawari from xxxHolic often make sacrifices for each other and some side characters end up falling into this as well.
Fushigi Yuugi: No matter how much crap and drama they go through, the Suzaku Seishi love one another to bits. This trope is shown in its extremity when Tasuki attempts to commit suicide to repent for nearly raping Miaka in the OVA while Brainwashed and Crazy..
Pandora Hearts: Alice at one point considers Oz, Gilbert and Break (and maybe Sharon too) to be this. It's quite cute. They demonstrate the fact that they're true companions left and right: saving Gil's hat, watching each other's backs, putting themselves at risk for each other, etc. And if you doubt Break or Sharon's being true companions, remember, it was Break who gave up that bell with the memories in it for Sharon's life without even flinching.
The Kenshin-gumi (Team Kenshin) from Rurouni Kenshin are obviously true companions. Megumi even pictures her family when she sees Kaoru and Yahiko arguing.
The Gekkostate in Eureka Seven, who refuse to leave when Holland attempts to disband it to protect them, because they're a family. Aw...
The main Power Trio of Zettai Karen Children starts out like this. In the beginning Minamoto has to make a lot of effort to gain their trust and eventually they start to include him.
Future GPX Cyber Formula's protagonist team, Sugo Asurada, are these towards the end of the TV series and the insert song in EP 17 drives the whole point home. However, it gets slightly toned down in the sequels, especially after the ZERO arc as the crew members are Demoted to Extra.
In Guyver, everyone who fights alongside Sho, especially Mizuki and Tetsuro who have been with him since the beginning. Even the Zoalords that aren't monsters form their own true companionship.
In Excel♥Saga, Excel's neighbors develop from three roommates to six close, albeit dysfunctional, true companions, with reluctant Team Leader Matusya trying to get her entire team, not just herself, out of as much trouble their boss throws them into as she can.
Angel Beats!, especially Otonashi and Hinata from the start of the series, but the SSS in general is pretty tight, especially by episodes 11 and 12 when their response to Otonashi telling them their belief about the afterlife is wrong is to simply give it a bit of thought, then all show up to fight with him anyway.
The "Afterschool Tea Time" of K-On! is a circle of very close friends first and a band second. They're so close that at the end of high school they all apply and ultimately get into to the same college, with Mio giving up a scholarship to a prestigious university to do so.
After performing at their final high school festival concert together, all five of them break down crying at the thought of being separated after graduation.
From what we can see the members of Death Devil, Sawako's high school band, were very much like their distant kouhai Afterschool Tea Time. Although working life has caused them to drift apart somewhat, they are still True Companions after all these years.
The cast of Rosario + Vampire definitely qualify, especially in the more recent chapters. Tsukune and the girls all care about each other very deeply, and have always been willing to go to great lengths for each other.
Takatsuki and Nitori in Wandering Son. They have other friends but they're not quite as close to them.
The crew of the Bebop from Cowboy Bebop, which makes their eventual dissolution all the sadder.
Pretty cruelly deconstructed in Rumbling Hearts. Takayuki, his girlfriend Haruka and their common best friend Mitsuki are this, though Mitsuki also has a secret crush on Takayuki... things are stable, aren't they? Well, Haruka barely survives an horrible accident but is left in a coma; when she wakes up years later... not only are Takayuki and Mitsuki together, but she suffers a bad case of Trauma-Induced Amnesia. At Haruka's doctor's request, everyone (not only Takayuki and Mitsuki, but also Haruka's family) decide conceal the truth that three years have passed among other things... and despite all of Mitsuki and Takayuki's efforts to keep Haruka's fragile mind safe (which includes them hiding their relationship), the consequences emotionally destroy everyone in the main cast... Haruka included.
The main cast from Doraemon, most notably in the movies.
The main character from To Aru Majutsu no Index suffers severe consequences due to his True Companions. Ironically it is not that they are in danger but because so many of them are well-connected, personally powerful or politically powerful themselves, Touma gets noticed by the wrong kind of people. The kind of people who have no qualms about murdering him to ensure his "faction" never coalesces into something formidable. To whit some of his True Companions include the Index Librorum Prohibitorum, a level 5 ESPer and a Saint.
The crew of the Super Dimension Fortress Macross, both civilian and military. When the civilians are denied the opportunity to leave the ship for Earth partway through the series, they are understandably angry, but counter with an attitude of "This ship is our home! We can live a happy life here too!" A couple random extras in one scene remark that people are beginning to identify themselves more as "people of the Macross" than their previous nationalities.
Team Fukuda in Bakuman。, composed of Fukuda himself, Mashiro, Takagi, Eiji and Aoki (Hiramaru's technically a member, but really just uses it as an excuse to take breaks from writing his manga whenever possible, and Nakai is one of the original members, but leaves eventually.) The group is formed by Fukuda to "change the face of Jump," which consists mostly of the team helping each other out with their respective manga while maintaining a healthy rivalry.
In Attack on Titan, Eren, Mikasa, and Armin have been true companions since childhood. Curiously, Eren doesn't seem to be fully aware of this and seeks out other true companions as well. He initially has a hard time believing that Annie, Reiner, and Bertolt are his enemies because he saw them as comrades. When the truth is shoved into his face, he doesn't take it well. Eren also started bonding with the Survey Corps members assigned to monitor him which ended badly after they were all slaughtered by Annie.
Inazuma Eleven is built around this through and through: no matter if a match is lost or a point is taken, the individuals come before the team and the team comes before a victory. Mostly prevalent in the first three seasons, but survives on multiple occasions in the following ones.