If you are doing the exam on Tuesday: the teachers at my school think its probably Crooks, Slim, Curley or Curley's wife. You will get a section from the book to analyse and then a question about themes you will have to answer both 15 marks each.
I would make a mind map or what ever you use to revise and go through the whole book to make notes about all of their important quotes and then analyse them depending on their appearance, how they treat people, how they affect people and how they speak.
Then the main question for Slim is "how does it represent ranch workers in the 1930's ?". Curley's wife "how were women treated in the 1930's?" and for Crooks "how were black people treated in the 1930's?"
Reread the book over this weekend and I will be adding character analysis tomorrow and Monday night and just search for help on get revising.