Marianne's redeeming feature really comes to the fore in chapter 16. It's her love for her family. In the middle of all her distress over Willoughby, in the middle of her massive disappointment that Willoughby has not returned, Edward Ferrars shows up and we have:

He was the only person in the world who could at that moment be forgiven for not being Willoughby; the only one who could have gained a smile from her; but she dispersed her tears to smile on him, and in her sister's happiness forgot for a time her own disappointment.