In book #3, it is revealed that she is allergic to strawberries. She and Eleanor like to play hide & seek, but they come out of their hiding place giggling before anyone could find them. In book #45, it is revealed that she has an imaginary friend named Jimmytony.
In book #54, it is revealed that Nina attends preschool three times a week and her preschool teacher is a male. Nina used to have a baby blanket called Blankie which she took to school with her & got teased because of it.
When Dawn Schafer babysat for Nina and Eleanor, Nina's blanket fell apart in the dryer, but Dawn puts the pieces of the blanket in Nina's shoe, under her sleeve, etc and tells her that she can take Blankie to school without being teased by the other kids.