The Spirit Riders are a nominal raider or "biker gang" consisting of Irathient outlaws and Ark Hunters. According to the "Pilot" episode they are reputed to hate Humans, though not as much as they despise the Volge (and, as a general rule, the Castithan). They tend to avoid cities and keep to the Badlands due to the unlawful nature of their reaving, but are not an isolated organization.
When the town of Defiance was founded eight years before the events of the "Pilot", it had a population of a thousand Irathients, but they rose up and took arms against the city's forces over the policy of mandatory vaccinations for all children. Under Mayor Nicky's orders deputies were going door to door vaccinating Irathient children by force. Some Irathients were killed and the rest left town. Some of the townspeople still look down on the Spirit Riders in the belief that they spread disease ("The Devil in the Dark"). Later, when a highly contagious disease outbreak occurred among the humans of Defiance, the Spirit Riders and other Irathients were believed to be immune carriers and forcibly quarantined in Rafe McCawley's mine by order of the town council. They were released once the cure was delivered (If I Ever Leave This World Alive).
Nolan and Irisa came to Defiance when their roller and supplies were stolen in a raid by Spirit Riders. Mayor Amanda Rosewater questions whether Irisa was abducted, as relations between the town and the Irathients are not at their best. Doctor Yewll determines that Irisa is not one of the Spirit Riders as she does not have their traditional tribal markings.