For the similarly named Vulcan, please see Koss.
Cos was a humanoid trader who acquired a gambling device that altered the laws of probability, so that those who won would experience good luck, and those who lost would experience bad luck.
In his youth, according to him, he had vigor, fame, and wealth. However, this changed when he acquired the gambling device. His trade deals, alliances, and every other endeavor he made all failed.
In 2370, he was being held in Deep Space 9's holding cells, when Odo placed Martus Mazur in the same cell as Cos. Soon after, Cos finally won a game in the gambling device, and immediately died, a contented man. (DS9: "Rivals")
Cos was played by Albert Henderson.
The name Cos originated from the source's script. The script refers to him at different times as "Cos" and "the Cos," making it unclear as to whether Cos is his name or his species.