The Arin'Sen were a pale-skinned humanoid species 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 Enterprise NX-01 encountered a severely damaged Arin'Sen vessel in a star system just outside Klingon territory. The surviving Arin'Sen told Captain Archer 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")