This Cossack sounds like mean version of Andre the Giant. He’s the ideal butler-bouncer: huge, strong, obedient, and dumb (literally). We don't get a ton of character development from this guy—he's basically just there to be the muscle.
And Zaroff puts him to good use as right-hand man and designated whipper. Consider the first time we meet him: “The first thing Rainsford's eyes discerned was the largest man Rainsford had ever seen--a gigantic creature, solidly made and black bearded to the waist. In his hand the man held a long-barreled revolver, and he was pointing it straight at Rainsford's heart.”
To be fair, the man is deaf and mute, but let’s not assume he’d have welcoming words even if he could talk. In short, not your typical meet-and-greet. And he's the alternative to the great hunt. If you're not up for a three-day game of cat-and-mouse, Zaroff is perfectly ready to let you be whipped by Ivan, who "once had the honor of serving as official knouter [whipper] to the Great White Czar, and he has his own ideas of sport.”
So, be chased by a psychopathic hunter or submit to the fun and games of Ivan the Whipper? Well, Rainsford manages to kill Ivan with the old Ugandan knife tied to a sapling trick. This loss really bums out Rainsford. Where is he supposed to get a lackey muscleman in the middle of the Caribbean? Monster.com?