When Rainsford knocks on the door of the chateau, he is greeted by
... 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.
Rainsford can see for himself that Ivan seems not to understand him; he finds out later that the giant is deaf and dumb. He wears a military uniform and has a military bearing, snapping to attention as Zaroff descends from the stairs. Zaroff later tells Rainsford that Ivan had formerly served
"... as official knouter (whipper) to the Great White Czar..."
Like Zaroff, Ivan is also a Cossack,
"... like all his race, a bit of a savage."
Ivan can read lips, since Zaroff
... spoke to him with lips that moved but gave forth no sound.
Any of Zaroff's prisoners who refuse to be hunted are turned over to Ivan. Ivan eventually joins Zaroff in hunting Rainsford, but he is used only as a last resort, leading the pack of dogs that are close on Rainsford's trail. Rainsford's last trap, with his knife attached to a sapling, does not kill Zaroff, who
... was still on his feet. But Ivan was not. The knife, driven by the recoil of the springing tree, had not wholly failed.