The novel's protagonist, Stanley is an overweight kid with a lot of bad luck. He is convicted of a crime he did not commit and is sent to the Camp Green Lake juvenile detention center. Non-violent and generally kind, Stanley has a difficult time in school and at the camp. Through his experience at the hellish Camp Green Lake he becomes physically stronger and more self-confident. He befriends Zero, another boy at the camp and through this friendship he succeeds in eradicating his family curse.
Zero is another camper at Camp Green Lake who becomes friends with Stanley. Zero is the best digger and generally thought to be stupid by the counselors and the other boys. In truth, Zero is very smart, although he has never been taught how to read. Zero is the great-great-great-grandson of Madame Zeroni, the woman who put a curse on Stanley's family. Zero has suffered many hardships in his life, even more than Stanley, but he never completely despairs and always shows incredible strength and willpower.
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X-Ray is the leader of the group of boys at Camp Green Lake. Although he is small and cannot see well, he manages to take charge and have the other boys follow his orders. He is nice to Stanley when Stanley gives him the gold tube that he finds in the dirt.
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Squid is another boy at the camp. He is as tough as X-Ray, although he often follows X-Ray's directions. Squid often taunts Stanley about receiving letters from, and writing to, his mother. When Stanley is allowed to leave the camp, Squid reveals that he also cares about his mother and he asks Stanley to call her and tell her that he, Squid, is sorry.
Another boy at the camp, Magnet earned his nickname by his ability to steal things. "My fingers are like little magnets," he claims after he steals Mr. Sir's sunflower seeds in Chapter 19.
Along with Squid, Armpit seems to be one of X-Ray's closest companions. He is the one that pushes Stanley to the floor when Stanley forgets to call him by his nickname. It is also Armpit who the Warden gets angry at when she realizes that the boys are not digging up what she wants in Chapter 17.
Stanley thinks Zigzag is the weirdest kid at Camp Green Lake. Zigzag is very violent and does not even apologize to Stanley after hitting him in the head with a shovel.
The Warden is the ultimate symbol of cruel authority at Camp Green Lake. She rewards only those who do what she wants and uses her power to threaten everyone else. She has hidden cameras that she uses to spy on the boys and nail polish with rattlesnake venom that she will use to scratch those who displease her. She is a descendent of Charles and Linda Walker and is making the boys dig holes in an attempt to dig up Kate Barlow's treasure.
Kate Barlow is the schoolteacher-turned-outlaw who robbed Stanley's great- grandfather. She lived on Green Lake one hundred and ten years ago, when there was still water in it.While intelligent and kind by nature, the murder of her beloved Sam turns Kate into a violent outlaw who lives on the wrong side of the law.
Sam and Kate were in love one hundred and ten years ago. Sam knew how to make many medicinal remedies out of onions and he was strong and smart. He also loved his donkey, Mary Lou. Because he was black, Sam suffered from racism, which prevented him from attending school or being with Kate. Sam is killed by the racist people of Green Lake after he and Kate kiss.
Charles is the son of the richest family in Green Lake at the same time that Kate Barlow is the school teacher. He is spoiled and stupid and gets upset when things are denied to him. When Kate Barlow refuses to go out with him he becomes very angry. It is Charles who leads the citizens of Green Lake to burn down the schoolhouse and kill Sam after he kisses Kate. Charles also has a very bad foot odor caused by a fungus. For this reason people call him Trout, because his feet smell like dead fish.
Elya is Stanley's great-great-grandfather and the reason that Stanley's family has such bad luck. After becoming disillusioned with the woman he thought he loved in Latvia, Elya travels to America, forgetting to fulfill a promise that he made to the gypsy, Madame Zeroni. Elya passes down generations of bad luck to his family. He also passes down the song that Madame Zeroni taught him while he was still in Latvia.
Madame Zeroni is the great-great-great-grandmother of Hector Zeroni, Stanley's friend at Camp Green Lake. She gave Stanley's great-great-grandfather a pig but when he broke his promise of carrying her to the top of a mountain she might have put a curse on his family.
The first Stanley Yelnats is the son of Elya Yelnats and the great-grandfather of the protagonist Stanley. Kate Barlow robbed the first Stanley Yelnats of his fortune while he was moving from New York to California. He survived by climbing to the top of a thumb shaped mountain (which he called God's thumb) which happened to be Sam's old onion field.
Mrs. Yelnats is Stanley Yelnats' mother. She does not believe in curses and always tries to point out the luck that the Yelnats' have had.
Mr. Yelnats is Stanley's father. He is an inventor who is smart and persistent, but unlucky. His attempts to discover a way to recycle old sneakers cause the Yelnats' apartment to smell very bad. Eventually he discovers a cure to foot odor and is able to hire the lawyer, Ms. Morengo to get Stanley out of Camp Green Lake.
Mr. Sir is one of the counselors at Camp Green Lake. He is tough and mean and is constantly eating sunflower seeds.
Mr. Pendanski is in charge of tent D, Stanley's tent at Camp Green Lake. He seems to be friendly but really he is just as mean as the Warden and Mr. Sir. This is revealed by his constant taunting of Zero and by his total lack of concern for the safety of Stanley and Zero when they are covered in deadly yellow-spotted lizards.
Clyde is the famous baseball player whose shoes Stanley is accused of stealing. He also suffers from the same foot odor that Charles Walker had. Clyde becomes the spokesman for Stanley's father's cure for foot odor.
Derrick is the bully from Stanley's school. The teachers never believed that Derrick could bully Stanley because Stanley was so much larger. It is Derrick's testimony, however, that eventually proves that Stanley is innocent of stealing Clyde Livingston's shoes.