A character sketch is a picture, through words, of a character. You want to include the character’s personality traits (what a character acts like) and physical traits (what a character looks like), as well as quotations to back up what you say. I will help you write one for these two interesting characters.
Sanger Rainsford is an interesting man. We know that he is an experienced hunter. Zaroff has read his books. We know that in the beginning, Rainsford is not empathetic with the animals he hunts. He is appalled by General Zaroff’s practices.
"Don't talk rot, Whitney," said Rainsford. "You're a big-game hunter, not a philosopher. Who cares how a jaguar feels?"
Rainsford is a curious man. He looks to see what the gunshot is, and that is how he ends up overboard. He is able to survive the hunt with a combination of cunning and experience. Despite his preference for human life, he does kill Zaroff in the end out of revenge and possibly to protect others from being hunted by him.
General Zaroff is described physically. You can get plenty of information from this description.
Rainsford's first impression was that the man was singularly handsome; his second was that there was an original, almost bizarre quality about the general's face. He was a tall man past middle age, for his hair was a vivid white; but his thick eyebrows and pointed military mustache were as black as the night from which Rainsford had come. His eyes, too, were black and very bright. He had high cheekbones, a sharpcut nose, a spare, dark face--the face of a man used to giving orders, the face of an aristocrat.
We also know that Zaroff does not consider human beings (lower class ones at least) more important than men.
"Life is for the strong, to be lived by the strong, and, if needs be, taken by the strong. The weak of the world were put here to give the strong pleasure. I am strong. Why should I not use my gift? If I wish to hunt, why should I not? I hunt the scum of the earth: sailors from tramp ships--lassars, blacks, Chinese, whites, mongrels--a thoroughbred horse or hound is worth more than a score of them."
Using these quotes, you can easily write a character sketch for General Zaroff and complete the one for Rainsford.