The narrator tells us that, just as there are children born with monstrous physical deformities, there are children born with monstrous psychological deformities; children in possession of a “malformed soul.” The narrator believes Cathy Ames is one of these monsters. Cathy was born with an innocent face and small, delicate hands and feet. She learns early that her sexuality can be used to manipulate people—though Cathy has no human sexual desire to speak of, she understand exactly how the desire of others can be used to her advantage. As a ten year old she frames two young boys for her sexual assault. When she attends high school, she drives her Latin teacher to kill himself. No one ever suspects her, though some people think there is a terrifying kind of emptiness in her eyes.
Cathy is a character study on the nature of evil. From the start we see that narrator John Steinbeck doesn’t conceive of her as possessed by inhuman evil impulses. Rather, she is evil and inhuman because she is incomplete—she is born with only the bad in her and none of the good. This description of her character reveals an important feature of Steinbeck’s worldview: evil is part of human nature. All people are born with bad in them. It is only when this innate evil is unchecked by goodness that it becomes inhuman. The idea that people are always both good and bad is one to which the novel will return repeatedly and at length.
Eventually Cathy grows tired of obeying her parents, and carries out a sadistic plot: she burns her house down with her sleeping parents trapped inside, and makes it seem as though she has been kidnapped. Everyone can see why a man would carry Cathy away—she was pretty and delicate.
Cathy benefits from her girly, innocent appearance. She uses gender roles and sexuality to her advantage—people invested in clichés and stereotypes, who are governed by their expectations, are more easily manipulated.