Whitney’s appearance is never described, but his personality is. Whitney is a minor character in this story. We do know some things about him though. He seems to be a bit of a philosopher. Consider this conversation about hunting.
"The best sport in the world," agreed Rainsford.
"For the hunter," amended Whitney. "Not for the jaguar."
"Don't talk rot, Whitney," said Rainsford. "You're a big-game hunter, not a philosopher. Who cares how a jaguar feels?"
"Perhaps the jaguar does," observed Whitney.
Rainsford does not believe that animals deserve consideration for their feelings. It’s probably a typical point of view for a hunter. You can’t kill an animal if you think of it as having humanlike qualities like feelings.
Whitney is not able to convince Rainsford to sympathize for the animal here, but he gets the thought into his head. Later on, Rainsford will definitely feel for them when he is in their position as Zaroff’s prey.
We also know that Whitney is superstitious. Most sailors are, and they are on a ship full of them. He shares with Rainsford many of the rumors about Ship-Trap Island while they pass it. Rainsford tells him that he thinks he is making matters worse by passing on the rumors, but he explains that the island is evil.
Sometimes I think evil is a tangible thing--with wave lengths, just as sound and light have. An evil place can, so to speak, broadcast vibrations of evil.
Of course, in some ways, Whitney is right. There is evil on the island. It is not in the island though. It is in the form of a person. General Zaroff is doing evil things there.
Whitney is an interesting character because the author uses him for foreshadowing. He gives Rainsford two very important pieces of information. He tells him that there is going to be a juxtaposition between hunter and hunted, and that something is very wrong on the island. Sometimes a minor character like Whitney can be used for very important purposes.