Cates is a modest, quiet, unpretentious 24 year old. The stage directions describe him as a "pale, thin young man" who is "not particularly good looking." He is a static character; a character who doesn't change throughout the play. Even though things happen to Cates, his character is the same at the end of the play as it is at the beginning.
Cates, who clearly represents John Scopes, is the defendant. Although they share the same role, that of defendant, Scopes volunteered to be arrested so that the Butler Law could be tested and never went to jail. The people in Dayton also never shunned Scopes; in fact, his teaching position was still open to him after the trial.
Cates spends the duration of the trial in jail. Never having been in any kind of trouble before, he is frightened, and, even though he is morally innocent and unrepentant, he is unprepared for the reaction of the townspeople. Because he is a proud man, he is embarrassed to have Rachel see him in jail. His strength of character is evident when he portrays a sense of humor to put her at ease. He jokes about the quality of food in jail as compared to the awful food at the boarding house where he's been living and the crime wave that would take place each summer if everyone knew how cool it was in the jail. When Rachel pressures him to admit that what he did (teach evolution to his sophomore science class) was wrong, Cates doesn't get angry or defensive. He simply tries to explain to her that "It isn't as simple as that. Good or bad, black or white, night or day." Cates is disappointed because Rachel doesn't understand what it means to think freely. Rachel is only aware of the fundamentalist beliefs that her father has forced upon her.
The circus-like atmosphere that surrounds the trial and the changes that Cates perceives in the people of Hillsboro both shock and sadden him. He "never thought it would be . . . like Barnum and Bailey (a circus) coming to town." Cates is observant and perceptive. People in the community treat him as though he is a murderer, as if he "had horns growing out of his head." Even his friends turn their backs on him. His reputation and respectability are slowly being destroyed. Despite his situation and the loneliness that he feels, Cates is determined to stand up for what he believes.
When Rachel testifies against Cates during the trial, he feels betrayed and scared because she reveals private conversations they shared. He is also angry because the truth is twisted in support of fundamentalism. Because Cates cares about Rachel, he puts his own interests and feelings aside and forgives her when he sees how distraught she is on the witness stand. He tells Drummond, "Don't plague her. Let her go."
Although the jury finds Cates guilty, he remains idealistic. In his statement to the court, he vows to continue to oppose the Butler Law in any way he can. He is proud of himself for standing up for academic freedom. He is also proud of Rachel when she arrives at the courthouse and shares her realization about how having the freedom to think is important. Cates and Rachel leave Hillsboro together.