Cecilia Rodriguez I believe that the antagonist would be leukemia, as the characters react to Mia in different ways after the diagnosis.
This answer contains spoilers… (view spoiler)[hi! I recently read this book, and I don't think there is an antagonist. Sure there are conflicts and fights, but there is no specific person who could be an antagonist. Because in the end Gyver and Mia end on good terms(really good terms). Ryan and Mia have conflicts but by the end they are almost all resolved. And the calendar girls all make up and are friends. so I don't think there is an antagonist. (hide spoiler)]