Miss Bates was the very person I was thinking of, Barbara! These and other characters are, in their own way, quite essential to the story.

On another note, Margaret, though not of the same nature as Mrs. Jennings or Miss Bates, is essential to the story too . Margaret reveals that Willoughby obtained a lock of Marianne's hair - I cannot see how any one else could have noticed this. She also brings up Edward Ferrars at dinner. I cannot see how anyone else could have done this either - but it is important as it has now excited Mrs. Jennigns' curiosity.