Cossack, nicknamed "Cossack the Terrible," is a Drule from Planet Doom who assumed command following Mogor's death at Prince Lotor's hands. He stands tall, comparable to Lotor but shorter than King Zarkon, and displays standard Drule traits: pointed ears, pupil-less yellow eyes, and no eyebrows. His long, unkempt brownish-grey hair is nearly always covered by his horned Doom helmet; he also wears a yellow cape.

Serving as an envoy and commander under Lotor, Cossack leads missions like the raid on Pollux, deploying Ultra Droids and taking hostages. During the Pollux attack, he destroyed a windmill, captured farmers, and announced himself as "Cossack the Terrible." He favors a whip, using it in combat and to torment slaves. While boisterous and jocular, his cruelty mirrors past commanders. He retreats when outmatched, as seen when he withdrew from Pollux upon Voltron's Lions approach.

Cossack shows deference to Prince Lotor, carrying out assignments directly for him while displaying occasional fear of Lotor's wrath. He shares moments of dark humor with Witch Haggar, once joking about kissing her in victory before retracting the statement. In the *Fleet of Doom* crossover, he aligned with King Zarkon and Viceroy Throk as part of the Fleet of Doom alliance against the combined forces of Lion and Vehicle Voltron.

Created exclusively for the U.S. *Voltron: Defender of the Universe*, he has no counterpart in the original Japanese *Beast King GoLion*. A deleted scene from *Fleet of Doom* and the final episode of the U.S. series' third season are the only times he appears without his helmet. His activities include operations after the Drule Empire's destruction, with his Pollux raid potentially occurring after the events of *Fleet of Doom*.