The main character in the story is Stanley Yelnats. Stanley is wrongly convicted of stealing a pair of sneakers and is sent to Camp Green Lake for eighteen months. At the beginning of the story Stanley is lonely, insecure, and lacking confidence, but by the end he is confident, independent, and has good friends. At the beginning of the story Stanley is described as "overweight," to the extent that "the kids at his middle school often tease him about his size." Stanley is also a very optimistic character, despite his seeming to suffer from peculiarly bad luck, and this optimism sustains him through the trials and tribulations that he endures at Camp Green Lake.
Another boy in the story, and perhaps the second most important character in the story, is Zero. Zero initially seems like a very strange character. He doesn't speak much, and he always seems to be angry or upset. Zero is uneducated and illiterate. It is for this reason that the other children call him Zero. Mr. Polanski tells Stanley that Zero is called Zero "because there's nothing inside his head." However, as the story progresses, we discover that Zero is smart in a more intuitive way. He is also very kind and generous, and he becomes very good friends with Stanley. Zero is also physically small but nonetheless strong and fit. He is described as "the smallest kid in Group D" and yet often "the first one to finish digging."
A third boy at the camp is Squid. Squid at first seems like an unpleasant boy. He teases Stanley when Stanley writes letters to his mother, and he threatens to punch Stanley after Stanley discovers him crying. Later in the story, however, we find out that Squid teases Stanley only in an attempt to distract from his own insecurities. Indeed we later learn that Squid very much misses his own mother.
A fourth boy at the camp is Magnet. Magnet's real name is Jose, but he acquired his nickname because of his predilection for stealing things. He says himself that his "fingers are like little magnets." Magnet steals a sack of sunflower seeds from Mr. Sir, but Stanley takes the blame.