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The Ousters are always in the background, and they have been for long before the book began: "The Ousters had been the single external threat to the Hegemony for [...] four centuries" (2.279). They're also known as "The Hegemony's prime bogeymen" (2.500). In fact, the whole background plot of the book seems to involve Meina Gladstone's plot to provoke the Ousters into attacking.
So what exactly are they? We only see them on-page in two chapters, but we learn that (1) They're a humanoid race of people who fled the Hegemony centuries ago; (2) They've been observing Hyperion for a long time for reasons unknown; (3) They've been evolving. They have "changed physically in three centuries (2.289). "Their long limbs, prehensile toes, and prosthetic tails would be added advantages for [zero gravity]" (2.325).
When the Consul is sent to spy on them, they induct him into their ranks and plan on using him to open the Time Tombs. Again, the purpose is unclear. The Consul betrays them, though. He shoots the scouts. But he opens the Time Tombs anyway, and action that will change everything.
The Ousters are ruthless warriors, but the same can be said about the "good" guys, the Hegemony. Who's the bad guy when both sides are at war? We don't know yet, but maybe we'll find out in the sequel.