It has often been stated that a typical plot device of JA's is to have an anit-hero, some one who distracts the heroine from the hero (i.e. Franck/Emma, Wickham/Lizzy, etc).
However, in re-reading chapter eight I was struck with a different thought. John Thorpe is not an anti-hero, IMHO, because Catherine, being a sensible person with taste, is not interested in him in the slightest. Rather, Isabella is (sort of) set up in this role. She distracts Catherine from Elinor, when it is importaint for Catherine (for her own personal improvment and the improvment of her relationship with Henry) to become friends with Elinor. Both friends have things in common, they're both older and more experianced then Catherine. Both are beautiful and love to read novels, but Elinor is the true friend and the better person, and Catherine must find that out.
Just a weird thought