We've talked a bit about Catherine being set up as a "non-traditional" heroine. However, I was very struck by Isabella's behavior during the course of Chapter 15:

"The very first day that Morland came to us last Christmas--the very first moment I beheld him--my heart was irrecoverably gone...I thought I never saw any body so handsome before."

"...My fortune will be so small; they [Mr. and Mrs. Moreland] can never consent to it. You brother, who might marry anybody!...As for myself, I am sure I only wish our situations were reversed. Had I the command of millions, were I mistress of the whole world, your brother would be my only choice."

Their conference was put to an end by the anxious young lover himself, who came to breathe his parting sigh....[James'] adieus were not long; and they would have been yet shorter, had he not been frequently detained by the urgent entreaties of his fair one that he would go.

I'm not exactly conversant in Gothic literature, but these strike me as the sort of things a "typical" heroine would say or do. Of course, in Isabella's case, it's all surface heroism--she has none of the sincerity or depth of feeling that would bring weight to her words. So Isabella is Catherine's foil--the former has the form of a heroine, but not the spirit, while the latter is the exact opposite.