Ivan is General Zaroff’s gigantic mute assistant.
When Sanger Rainsford falls off his yacht and ends up on Ship-Trap Island, he thinks the island is uninhabited at first. He is surprised to come across a ritzy chateau that definitely has a resident.
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.
This is Ivan, the giant man employed by General Zaroff. Ivan is a complete surprise to Rainsford. For one thing, he doesn’t talk. He seems to be a mute. Second of all, as it says above, he is huge. In addition to being huge he is very strong.
Ivan is basically General Rainsford’s assistant. He does whatever he is asked. He acts as a butler and also assists with the more unseemly duties of the island, like helping with the prisoners. Zaroff threatens Rainsford with Ivan, saying that Ivan will beat him if he doesn’t engage in the hunt.
If he does not wish to hunt, I turn him over to Ivan. Ivan once had the honor of serving as official knouter to the Great White Czar, and he has his own ideas of sport. Invariably, Mr. Rainsford, invariably they choose the hunt."
A knouter is someone who is in charge of flogging criminals until they die. It is supposedly something that Ivan enjoys. When Zaroff gives Rainsford the choice, he chooses hunting.
During the hunt, Ivan also assists Zaroff. For instance, he holds the pack of dogs. When Rainsford designs his trap involving the knife tied to a “springy young sapling,” it is Ivan who is hit by it, not Zaroff. Zaroff is disappointed, feeling that is would be “difficult to replace” Ivan, but no matter. The game continues.
Although General Zaroff is the main character of the story, Ivan is important too. You might possibly conclude that Rainsford might not have been able to kill Zaroff if he had not killed Ivan first. Ivan was acting as a kind of bodyguard for Zaroff, and while killing Ivan first was an accident, it did allow him to get to Zaroff. The story would have been very different without Ivan, and would have ended differently if he had not been killed.