As there are three story lines in this book (one main one and two subplots on other time lines), you need to first study the characters who know Stanley, the main protagonist. After that, make a separate list of the people associated with Stanley's ancestor who didn't take the pig up the mountain and then the ones involved with "Kissing Kate."
Apart from his immediate family members, other people very present in the life of Stanley are the Warden, Mr Sir, Mr Pendanski, Stanley's best friend Hector Zeroni (Zero) and the other boys digging holes at Camp Green Lake. This gets a bit complicated as each boy has been dubbed with a nickname which suits his physical appearance or personality. For example, since Stanley immediately stood up to a bully at the Canteen, the other boys admired his courage and nicknamed him 'Caveman.'
As the list is quite long, I will not enumerate all the characters here, but you can click on one of the following reference to get this information.