When we first meet Willoughby, he is described as carrying Marianne home in his arms. He is described as gallant, heroic, and handsome, the picture of an ideal romantic hero. This early description certainly does not give any clue to his true nature. When he first calls on Marianne, she sees that he is sincere and straightforward, with a happy, energetic nature. She also learns that he is fond of dancing and music and that, "Their taste was strikingly alike. The same books, the same passages were idolised by each." He is also described as being as equally passionate about interests as Marianne. Furthermore, through his daily visits, Marianne and the family soon learn that he is polite, intelligent, imaginative, and has "lively spirits, and open, affectionate manners" (Ch. 10).
None of these characteristics give us any clue to his extravagant nature and want of principles. However, we know from earlier descriptions of both Elinor and Marianne that Elinor has sense, and Marianne, while she may have it too, doesn't use it. Therefore, Austen very carefully prepped us to very soon understand that Marianne is wrong. While she is right about these positive traits and right to think they are admirable, she is wrong to think that they are all that is needed. They are not enough. We actually need these qualities plus principles and sense, just like Colonel Brandon has.