Characterization describes what exactly a character is like, who the character is as a person. Authors can use many different techniques for characterization, including dialogue, descriptions, and even other characters' thoughts about the one character. One character Austen characterizes well is Willoughby. At first glance, we think he is the perfect gentleman, the perfect hero, but if we look deeper at Austen's subtle characterization clues, we see that she actually depicts him as untrustworthy from the very start.
When we first meet Willoughby, Marianne has just fallen and twisted her ankle while running down a steep hill. Willoughby, who had been hunting in the area with his two pointers, appears at just the right moment and carries her home to safety. The fact that he whisked her away "without further delay," even though Marianne was too modest to accept his help, portrays him as the perfect benevolent hero. Not only that, we soon learn that he is "uncommonly handsome" and very charming (Ch. 9).
However, as charming as he first appears to be, if we notice other subtleties, we can see that Austen has actually characterized him as being mysterious and possibly even untrustworthy and devious. One characterization technique Austen uses is setting. It's not coincidental that Marianne meets him just after it has been raining and on a day when it threatens to rain still. The winds were even blowing harshly from the southwest. It even starts raining full force while Marianne and Margaret are climbing the downs, making them run to the house. Hence, Willoughby appears when it's raining, and rain can be symbolic of emotions. More specifically, the rain can symbolize the heartache that Marianne is soon to suffer due to Willoughby. Willoughby not only appears in the rain, he leaves in the rain, which the characters agree make him seem very mysterious, as we see in Austen's line, "He then departed, to make himself still more interesting, in the midst of a heavy rain" (Ch. 9).
Hence, we see that while Austen portrays Willoughby as charming and gallant, she also characterizes him as a bit dark and mysterious, which foreshadows that his character will soon be seen as untrustworthy.