Who is Isabella Thorpe from Northanger Abbey and what is their importance?

Isabella Thorpe is John's brother and the girlfriend—then fiancé—of James Morland. As a casual friend, Isabella is perhaps an agreeable sort. She is beautiful and wily, draws the attention of young men, and knows a great deal about dress, fashion, and popular culture—though her knowledge is not deep. Upon first meeting, Catherine and Isabella enjoy each other's company at the social dances and other functions. But even from the first days it is evident that Isabella is more style than substance and cares far more for herself than for anyone else.