Those characters whom Austen designs to be living by folly are certainly consistent at it! Yet there are other characters who combine elements of folly with sense who, from a casual reading, may appear to present an inconsistent picture of folly--because they are inconsistent, but only by Austen's design and intent. First of all, the definition of folly in the sense used means foolishness, stupidity, rashness as a state of personal being (an individual characteristic) or in regards to an action, idea, belief etc. (derived from the Collins and Random House Dictionaries on Dictionary.com).
Sir Walter and Elizabeth are the two main characters who embody folly. An example of Sir Walter's folly, that he consistently upholds throughout, is his inability to be realistic about living expenses because he connects appearances of gracious living with his identity as a baronet (which is the lowest class of hereditary titled commoner and ranked just above nonhereditary knighthood):
What! every comfort of life knocked off! Journeys, London, servants, horses, table--contractions and restrictions ... No, he would sooner quit Kellynch Hall at once,
Other examples are the extraordinary power and importance Sir Walter attaches to the use of Gowland face lotion (recommended to Anne and Mrs. Clay: “should recommend Gowland, the constant use of Gowland”) and his silly, unseeing. all-encompassing vanity.
Elizabeth's primary claim to showing unrelenting folly is her insistence that she must marry only a man who has a baronet that is "worthy" of her own station in life. As a result of this folly (i.e., stupid idea or belief), she has been the marriage-eligible belle of the social set balls for thirteen years while her rejected suitors have gone on to marry ladies displaying far less folly:
Thirteen winters' revolving frosts ... thirteen springs shewn their blossoms, ... she had the consciousness of being nine-and-twenty.
The Dowager Viscountess Dalrymple and her daughter, the Honorable Miss Carteret, appear only briefly but give another very good and consistent picture of folly (“Anne was ashamed. ... [They had] no superiority of manner, accomplishment, or understanding”), which leads to consideration of the primary character who inconsistently demonstrates folly, cousin William Elliot. At times, William can show sense and reasoning power but his major decisions in life are based either on maliciousness (which we aren't concerned about here) or on folly. The prime example of this is his elopement with Mrs. Clay--the scheming, conniving, Mrs. Clay who is herself a good example of inconsistent folly: “[whether] Sir Walter ... may not be wheedled and caressed at last into making her the wife of Sir William.”