Ivan serves as Zaroff's right hand man. Ivan and Zaroff are both Cossacks and served in the military in Russia. He answers the door when Rainsford arrives at the chateau, he waits on Zaroff and Rainsford at the dinner table (waiter), he shows Rainsford to his room and lays clothes out on the bed for him to wear to dinner (butler), and he was Zaroff's partner on the hunting expedition to kill Rainsford. Zaroff seemed to consider Ivan a weaker person because he was deaf and dumb. He made a good partner for Zaroff since they had both served in the military.
Ivan is an enormous man who not only acts as a sort of butler to Zaroff. He answers the door to the lost on the island and makes sure that they are well taken care of. His brings out the food and prepares the rooms for guests. He watches over the latest captives and prepares Zaroff and the others for the hunts. Ivan is not just a house boy to Zaroff, he is employed to do anything that is asked of him including torturing and killing any person who does not agree to play the game.
Ivan ends up meeting his demise when Rainsford sets a trap for him because Zaroff asked that Ivan help him hunt down this worthy adversary.
Ivan is a huge man that scares a person just to look at him. As Zaroff's servant, he does the dirty work for Zaroff. It's his responsibility to take care of the sailors who don't wish to participate in the hunt. I'm sure Ivan does just about everything for Zaroff, including finding sailors for Zaroff to hunt. His duties aren't completely spelled out, but since he's the only other person living with Zaroff, he must do anything and everything that Zaroff wants and needs done. Rainsford accidentally kills Ivan when Zaroff brings him back with him to continue the hunt for Rainsford.