Who is Aunt Dinah from The Life and Opinions of Tristram Shandy and what is their importance?

Aunt Dinah is a source of embarrassment for the Shandy family as she married a lowly coachman. However, much to Toby's disgust, Walter loves to recount the tale to guests and actually inherits a large amount of money from her at the end of the second volume. Though he never meets her, Tristram thinks she one the only woman in the family with character.