... who is used to having her own way, even with her guardians (as pointed out in a thread below); we are told that "Mr. and Mrs. Ellison would not be sorry to have Miss Grey married, for she and Mrs. Ellison could never agree."

So, this smart, stylish, exceedingly rich young woman who gets whatever she wants, even out of the supposed authority figures in her life, decides that she wants Willoughby. Why, you ask?

Why indeed! He's gorgeous, he's charming, he has the gift for making everyone like him wherever he goes -- why ever NOT? The perfect arm-candy for an heiress like Miss G!

And, precisely beCAUSE he's not rich, that means that she'll be able to boss him around (as, indeed, she does in the scene where we first meet her, with a single glance of an eye!), which to someone like that makes him even MORE desireable.