One heroic quality is, I guess, consistency. Jane Austen seems to set Isabella's situation up to be heroic, but her actual description is that of a flighty, silly girl. Her first words, in reply to Catherine's apology for her lateness and her hope that Isabella hasn't been waiting long, are:

"Oh! These ten ages at least. I am sure I have been here this half hour". [Chapter 4]

And the chapter finishes:

"...to show the independence of Miss Thorpe, and her resolution of humbling the [male] sex, they set off immediately as fast as they could walk, in pursuit of the two young men."