Been a minute since I last read it, but Whitney is that guy Rainsford is traveling with at the very start, right? They're on a yacht, heading somewhere for a hunt, and they're talking about the animals they're going after. Whitney says something about the jaguar feeling fear, understanding what it's like to be hunted. Rainsford just brushes it off, says it's nonsense.
To me, that conversation is the entire point of his character. He's there to set up the whole moral argument Rainsford will have to live through later. Without Whitney's bit of philosophy, Rainsford's whole experience on the island wouldn't hit as hard. It's foreshadowing, but it's also the only voice of empathy we hear before everything goes sideways.
Whitney basically disappears after the first few pages, but his words stick around, hanging over the rest of the story. I always wonder if Rainsford remembered that chat while he was running for his life.