A foil is the near complete opposite of the main character (whichever character they want you to find a foil for).
Rainsford and Whitney were good hunting friends with numerous similar interests. They could not be foils because of how close in similarity they were. Even when they disagreed on how animals felt about being hunted, Whitney seemed open to and intrigued by Rainsford's points and way of thinking.
Ivan is a near irrelevant character, being a mere Cossack who follows whatever General Zaroff says. He is mindless and has almost zero traits to even compare to Rainsford, let alone any traits aside from a mindless follower to begin with.
The answer would be General Zaroff. This is almost like the cliche protagonist vs antagonist foil. Both of them are hunters, but different kinds. Zaroff got bored with animals and wanted to hunt human people instead, whereas Rainsford had enjoyed the thrill of an animal hunt and thinks that the hunting of people is murder. Zaroff is more heartless and cold, a killer, if you will. Rainsford seems to think highly of actual people, and had no interest in playing Zaroff's game.