[SPOILER] Originally portrayed as enemies of the Hegemony and of humanity early in the two Hyperion novels, they are later revealed to be peaceful beings of great technological advancement and of benign, albeit bold, intentions for the future of the human race. Nonetheless, they are foe to both the Hegemony and later to the Pax throughout the four books.
Having adapted to numerous different environments (both on- and off-world), the Ousters are very diverse and exist in a wide variety of forms. Some Ousters resemble baseline humans in appearance, with the exception of having developed elongated limbs and prehensile feet; others have adapted to their environment by developing furred coats, webbed fingers or other augmentations.
Despite their wildly divergent appearances, all Ousters are genetically human - that is, they do not differ sufficiently from baseline humans to be considered a separate species, and could theoretically reproduce with "normal" humans. However, it is likely that the Ousters' continued evolution will result in them diverging even farther from baseline human stock, to the point that they will form an independent species entirely. Ousters of all kinds may make use of electromagnetic wings spanning hundreds of kilometers as they ride the solar winds.
Ouster Angels (True Ousters)
- Main article: Ouster Angels
There is one subset of fully space-adapted Ousters , also known to outsiders as "Angels," live in microgravity environments such as orbital forest rings alongside other types of Ouster. They are some of the most radically-adapted Ousters, being fully adapted to the vacuum of space, and have an array of bio-technological enhancements that allow them to survive in such environs, such as embedded electromagnetic wings, bio-radio transmissions, forcefields such as over their mouths, and even the ability to feed off sunlight. 
Having spent centuries living in open space, much of the Ousters' technology is designed to make optimum use of low-gravity conditions. Ouster soldiers frequently wear suits equipped with prosthetic tails that can serve as weapons or grasping limbs. Their boarding craft are likewise equipped with tentacle-like cable arms that grip the hulls of target vessels.
The Ousters have achieved several remarkable feats of bioastroengineering, including orbital tree-rings and the Startree Biosphere. Their home swarm itself is a marvel of megascale engineering, consisting of a number of asteroids that have been encapsulated in containment fields, terraformed and linked to one another by containment field tubes. In order to allow travel between the different asteroid habitats, streams of water run along some containment field tubes, allowing Ousters to ride ferries to their destination.
The Hegemony was unaware of the true nature of the Ousters' homeworlds and advanced habitats, and many perpetuate the belief that the Ousters live in spartan "can cities" or similar space station habitats. It regarded the Ousters as barbarian hordes who had left Sol System in what they though was a crude fleet of leaking "O’Neill cities", tumbling asteroids, and experimental comet farm clusters.
The Ousters are strictly against the use of farcasters as a means of transportation, which means that they must travel at relativistic velocities between planets and star systems; this method of travel, though far more time and resource-consuming, nevertheless avoids disturbing or damaging the Void Which Binds, and the entities living within it.
An Ouster Migration Cluster is the Hegemony name for a group of ouster vessels as they travel between the stars. Migration clusters can range in size of up to over 4,000 ships. The ships themselves can range in size from single person ramscouts to can cities and comet forts (or more accurately, terraformed asteroids) transporting tens of thousands of Ousters.
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The Ousters' origins can be traced to the early period of human interstellar expansion and colonization. Numerous slower-than-light seedships were sent from Earth to distant star systems, with their occupants stored in cryogenic fugue to be awakened upon arrival. The passengers of these ships were augmented with internal nanotechnology, allowing them to rapidly evolve and adapt to foreign conditions of other worlds.
As time went on, the relationship between the Ousters and Hegemony became increasingly uneasy. Armed conflicts would take place in areas far into the Outback. This culminated upon the Hegemony declaring all-out war on the Ousters as they mistakenly thought that Ouster attacks upon multiple Web Worlds were imminent.
The various forms of Ouster continue their way of life long after the fall of the Pax, becoming ever more specialized for their varying habitats.