Isabella Thorpe might just be the most entertaining person in the entire novel. You might say, "But Isabella is manipulative and scheming and unfaithful and self-serving and lots of other not-so-nice things!" To which we say, true. But Isabella is fun and unpredictable and very funny. Isabella is one of the types of characters that Austen does best: the young woman who behaves scandalously and is wildly entertaining and even hilarious while doing it.
So Isabella fits in with a whole host of scheming women of "questionable morals" that populate Austen's works, including Lydia Bennett of Pride and Prejudice. Isabella is also rather timeless. If she existed today, she'd be the star of a movie like Mean Girls as the manipulative 'friend' who causes trouble for the naive protagonist. And this is exactly what she is doing in Northanger Abbey.
Isabella raises confounding others to an art form. Though it does help that her targets are generally lacking in the mental shrewdness department (we're looking at you, Catherine). Isabella is as coy and artificial as Catherine (and James, for that matter) is genuine and trusting. If James is taken in by Isabella's good looks and thrown off balance by Isabella's ceaseless flirty chatter, then Catherine is dazzled by Isabella's bold personality and overt declarations of affection and friendship.
One of Isabella's main tricks seems to be transforming, chameleon-like, into whatever others expect her to be. She expresses a desire to live in the country on a small income because James agrees with that sentiment. She makes sure to agree with whatever Catherine says, even doubling back and contradicting herself in order to make her opinions align with Catherine's.
Take a conversation between Isabella and Catherine, where Isabella rapidly shifts gears and continually makes it seem like Catherine is saying things that she is not.
First, Isabella bashes the Henry Tilney on Catherine's behalf, supposedly:
"Let me entreat you never to think of him again, my dear Catherine; indeed he is unworthy of you."
"Unworthy! I do not suppose he ever thinks of me."
"That is exactly what I say; he never thinks of you. – Such fickleness!" (16.5-7)
By deliberately misinterpreting what Catherine says, Isabella manages to both agree with her and undermine her simultaneously. Later in the conversation, Isabella deploys her skills in linguistic confusion once again to make something seem like Catherine's idea. Catherine starts, saying:
"Well, I shall see how they behave to me this evening; we shall meet them at the rooms."
"And must I go?"
"Do you not intend it? I thought it was all settled."
"Nay, since you make such a point of it, I can refuse you nothing. But do not insist upon my being very agreeable, for my heart, you know, will be some forty miles off." (16.10-13)
Isabella can talk circles around anyone and Jane Austen has her do exactly that. Rather than provide us with insight into Isabella or extensive descriptions of her personality, Austen lets Isabella present herself to the reader. The effect of this is quite ironic, as we understand the scheming Isabella far better than the naive Catherine does.
But Isabella is not some sort of evil mastermind. In fact, her schemes generally fail, making her more funny than threatening. Isabella is the Wile E. Coyote to Catherine's ever-fortunate Road Runner. And Isabella meets her match in Captain Tilney, who seduces Isabella and then leaves her in a lurch with little hope of forming another engagement after such a scandal. Though the novel glosses this over, Isabella's ultimate fate is not all that funny. After all, Isabella isn't exactly a villain in the twirling-mustache, evil laugher sense. She's much more of a grey character, someone of rather questionable morals and methods. She's using all the limited tools at her disposal to navigate the minefield of polite society, much like Becky Sharp of Vanity Fair (a mid-nineteenth century novel by William Makepeace Thackeray).
In fact Isabella is a rather interesting mirror of Catherine. The two are foils in many respects. Isabella's artifice highlights Catherine's artlessness, and so on. The two also share some fascinating personality traits. At the hands of Captain Tilney Isabella proves as easy to manipulate and as prone to faulty assumptions – namely regarding Tilney's desire to marry her – as Catherine generally is.
And Catherine often displays a degree of self-centeredness that characterizes Isabella. While Isabella morphs into whatever others would like her to be in order to manipulate them, Catherine often transforms everyone around her into carbon copies of herself, assuming their motives and interests are like her own. Arguably, Isabella is just selfish and manipulative while Catherine is just inexperienced. But there is a degree of self-centeredness in both women that make for some interesting parallels and comparisons.
Isabella is not only entertaining, she's also very complex. And given that she is often lying and playing word games with other characters, readers definitely have to work at understanding her fully.