"Holes" is such an amusing book with some unusual characters. Some of the primary characters in the book are.
Stanley Yelnats IV is the main character. He has no luck but bad luck and it runs in his family. A curse was placed on his family by Gypsy Madame Zeroni because of his "no-good-dirty-rotten-pig-stealing great-great grandfather".
Gypsy Madame Zeroni si the one who curses Stanley's family so that the curse can only be lifted when a favor is paid back to one of her ancestors.
Hector Zeroni (Zero)becomes Stanley's best friend. Zero can not read and climbs into one of the holes daily that he and Stanley have to dig so that Stanley can teach him how to read. He also turns out to be Madam Zeroni's great, great, great grandson. Since Stanley helps the boy the curse ends up being lifted.
X-ray is a dominating boy at camp. He is smaller than the other boys but has a leadership ability about him. He wears glasses. He is jealous of Zero's friendship with Stanley.
The Warden is the operator of the boys camp where the boys are incarcerated and forced to dig holes in search of a lost fortune. He is cruel and selfish and subjects the boys to long hours of digging in the hot sun.
Katherine Barlowis a character from the past. She is a pretty school teacher who falls in love with a black man. After Sam is killed Kate becomes angry which leads her to become the outlaw Kissin' Kate Barlow.
Sam is an onion seller who also loves the girl but is doomed because it is against the law for a black man to kiss a white woman. Sam is killed.
Stanley's father is a man who is kind but a little like a nutty professor who is always trying to invent things.
There was a lot of main characters including Stanley, Zero, Sir, X-Ray, and etc. The reason is because the story is based on a friendship and trust theme.
as an add on to the answer above Sir is all so a very important character he is the one of the rule inforcers at camp he provides the water for the boys when they're digging.
i hope i helped