Stanley Yelnats is a great rounded character -- he changes throughout the course of the novel. He changes physically while he’s at Camp Green Lake because digging holes in the hot sun all day will help any person lose weight and grow stronger. Mr. Pendanski likes to point out the purpose for digging holes as something completely innocent: “You’re not looking for anything. You’re digging to build character” (page 27).
There are other characters who don’t change at all, flat characters like the Warden, Mr. Sir, and Mr. Pendanski. They don’t change their beliefs, they continue to act intimidating or cruel or selfish throughout the novel. Stanley, however, does change. He changes his mind about helping Zero learn to read and he accepts his fate by recognizing that he ended up at Camp Green Lake for a reason and he was a better person for it by the end of the novel: “If Zero had just kept those shoes, then neither of them would be here right now. As Stanley stared at the glittering night sky, he thought there was no place he would rather be” (page 187).