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Cos was an alien 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 this device. His trade deals, alliances and every endeavor he made, 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 won a game, and immediately died, a contented man. (DS9: "Rivals")

Cos was played by Albert Henderson.
His name comes from the 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.