In the book Stargirl, why does Hillari Kimble say Stargirl isn't real?

's
novel, , tells the story of an unusual girl named Susan, though she prefers
to go by "Stargirl." A new student at Mica Area High School, Stargirl's eccentric
clothing and odd behavior garners all kinds of attention and affection, especially that of the
narrator, Leo. 
 
While almost everyone is curious about Stargirl, Hillari
Kimble, one of the more popular girls in the school,...