Whitney is Rainsford’s hunting partner, and he gets about, oh, three lines in the entire story. His main role is to set up Rainsford's character by being a contrast. Whitney believes that jaguars and prey in general experience “The fear of pain and the fear of death,” and therefore deserve a little sympathy.
But he goes downstairs before Rainsford falls over the side of the yacht, and then presumably heads off to hunt those poor, sympathetic jaguars.
Hope that helps :)
