True Companions/Tabletop Games

Board Games

 * Despite the setting, Warhammer 40,000 even manages to demonstrate this trope with the Space Marines. Nothing is more important to them than loyalty to their Battle Brothers and fealty to the Chapter, and aside from the God-Emperor himself and the founding Primarch they recognise no authority other than their Chapter Master. This is why the Horus Heresy is considering so tragic, as conquering solar systems and crushing civilisations was perfectly fine, but turning against and fighting your brothers was inconceivable.

Tabletop RPGs

 * The werewolves of both Werewolf: The Apocalypse and Werewolf: The Forsaken follow the combined social instincts of humans and wolves to form small "packs" with each other, ideally a Five-Man Band. These packs follow the entire true companions trope: packmates are practically family, you might love or hate them, and intra-pack romantic relationships are considered incestuous. (But then, in The World of Darkness games, any werewolf/werewolf relationship effectively is incestuous, as werewolves must mate with humans—or, in the Old World of Darkness, wolves—or breed twisted, sterile mutants.)
 * New World of Darkness-specific:
 * The same goes, to a lesser extent, for most of the other supernatural groupings (Mage cabals, Promethean throngs, Changeling motleys, Hunter cells). Which, given the often cathartic nature of the supernatural societies, makes a lot of sense. Vampire coteries, on the other hand, tend to be brief marriages-of-convenience, formed by the recently Embraced until they get to grips with Vampire existence. The line about coteries is Jossed, sort of, in Clanbook Ventrue. As part of a section on how The M spreads, the author talks about how young coteries drink each other's blood to form bonds deeper than family. Young Ventrue, on the other hand, are discouraged from such things.
 * Kindred Cyclical Dynasties are another good example. Closer than family, often to the point where the lines between them begin to blur, cyclical dynasties are made up of two or more kindred, with the eldest acting as mentor to the next eldest, who acts as a mentor to the next eldest, etc. When the eldest falls into torpor the next eldest takes over, secure in the knowledge that his dynasty-mates will take him under their wing when he wakes up confused and isolated in decades or centuries.
 * Shadowrun tends towards extremes. Groups of runners who are largely indifferent towards each other tend to drift apart. Most established teams are held together by tight bonds. It's also not uncommon for every member of the team to have a plan to kill any or all of his teammates, just in case.
 * In Exalted, this is the idea of Solar, Lunar, Sidereal, and even Abyssal circles, plus Terrestrial sworn brotherhoods. As for when it doesn't turn out like that... well, just ask the Deathlord Eye and Seven Despairs about that one.
 * Adventuring parties in Dungeons & Dragons are usually this, for the practical reason that, for the game to work, the PC's have to stick together. "Splitting the party" is a phrase that makes most players and DM's cringe.
 * It's not really an exaggeration (it might even be an understatement) to say that Thri-Kreen society revolves around this concept, as well.
 * 7th Sea has the concept of "Rucken," two fighters who trust each other completely. Players can purchase an Advantage of the same name to gain an unusually powerful (100 Hero Points instead of the usual 75) NPC companion, with the catch that if the player ever abandons or betrays his Rucken, the Rucken becomes his sworn enemy (denoted by gaining the Nemesis background at its most dangerous level).