Cathy is first spoken about by M&M and Bryon around seven pages into the first chapter, which is page 7 in my copy. She first actually appears around six pages into Chapter 2, which is page 23 in my copy. She is the waitress at the hospital snack bar, and Bryon doesn't recognize her at first.
The copy I am using is the 1971 edition printed by Dell's Viking Press. Keep in mind that page numbers differ in different editions of the book. The chapter descriptions may be more helpful to you.