An ailing old alien gambler in the episode Rivals who shares a holding cell with Martus Mazur and owns the small device that alters the laws of chance.
Cos is an elderly alien seen in the second-season episode Rivals. Odo has locked him in a holding cell on Deep Space 9, and he is joined there by Martus Mazur, an El-Aurian arrested for swindling an old couple.
Cos has with him a little spherical gambling toy. He tells Martus that it has brought him nothing but bad luck, that he has lost everything, and that he has never once won with it. Shortly afterwards he wins for the first time, and dies.
Martus keeps the device and finds that his own fortunes improve at once. He has larger copies built for a club he opens to compete with Quark's, and the machines' effect on probability then spreads misfortune and unlikely luck all over the station.