The Arin'Sen were a pale-skinned humanoidspecies with a ridge running along their necks, vertical stripes running along their heads and two circular parts of the head next to the ridges.
In 2153, the starshipEnterprise NX-01 encountered a severely damaged Arin'Sen vessel in a star system just outside Klingon territory. The surviving Arin'Sen told CaptainArcher that, several years previously, their colony, Raatooras, had been annexed by the Klingon Empire. The Klingons had offered them protection, but abandoned the colony after stripping it of all valuable resources. Short on food and fuel, the Arin'Sen fled the colony, pursued by Captain Duras of the IKS Bortas. Having branded the Arin'Sen refugees as rebels, the Empire sent Duras to retrieve them, but Captain Archer refused to hand them over. After engaging in battle with the Bortas, Enterprise managed to evade Duras, and relocated the Arin'Sen refugees on a planet unknown to the Klingons. (ENT: "Judgment")
This species' name originates from production sources, such as the production report(X) released for "Judgment". David A. Goodman, who co-wrote "Judgment" and scripted the episode, named the species after Michael Aronson, who had been a friend of and fellow Trekkie with Goodman in college before becoming a writer for the New York newspaper Daily News.
David A. Goodman wanted the Arin'Sen to be escaped slaves. He ultimately thought that the representation of their past relations with the Klingons was softened and that it would have been better if the Arin'Sen had indeed been slaves who had escaped from a Klingon-dominated site, increasing the accuracy of calling them "rebels." ("Judgment" audio commentary & podcast)