This character appears at the beginning of the novel after Tom Joad meets him. Exactly after this meeting, Casy decides to move with the Joads family.
Jim Casy is a reverend in this book. But he is not a typical one with only advantages in his behavior. He also has weaknesses. Talking about strengths, he builds his philosophy, fights for his mission. And that deserves to be honored, not each person can stand for his beliefs. Talking about weaknesses – he was an immoral preacher who gives up living this life the way he used to live, he is isolated from humanity which is bad for a person of such a job.
He was a reverend who, ironically, succumbed to temptation too often and soon he left the ministry with disappointment. From the point of the modern person’s view, we can summarize that maybe he was making sins because of the Satan who changes him. Nevertheless, it wasn’t said in the book, so let’s believe that Casy has just changed.
Being confused about his controversial beliefs (I mean the ones between what is sinful and what is holy) he has renounced his calling. When he decided to travel to California with Joads family, he plans to change, to listen to all people and help them with their problems as much as he can. Casy is the spokesman for the author's main theories, including the multi-faceted themes of love and strength in unity.
Jim Casy in the Essays