Willoughby is presented in such a way that the reader likes him but is suspicious of him, and then has those suspicions confirmed (mostly).
Although Willoughby almost always appears to be a caricature, he sometimes develops depth. He seems like a gentleman, then a playboy; he’s perfect, and then cruel; he’s harsh, and then apologetic. Every time we think we have him figured out, he changes on us.
When Willoughby first appears, he makes a grand entrance and Elinor and her mother are smitten.
[The] eyes of both were fixed on him with an evident wonder and a secret admiration which equally sprung from his appearance, he apologized for his intrusion by relating its cause, in a manner so frank and so graceful that his person, which was uncommonly handsome, received additional charms from his voice and expression. (ch 9)
Sir John seems to have a good opinion of Willoughby, because when the women ask about him he says he is going to ride over and see him. He gives quite a character reference for him, calling him "As good a kind of fellow as ever lived, I assure you” and “he is very well worth catching” because he is about to inherit property.
Willoughby “was exactly formed to engage Marianne's heart” (ch 10). He was handsome, witty, kind, and romantic. The way they met was even appropriate, since he saved her like a damsel in distress. Of course, no man is perfect. Yet Willoughby turns into a villain, leading Marianne on and then dumping her in a letter.
Yet is Willoughby really a cad? Did he have them entirely fooled? Was he making a fool of Marianne, or did he genuinely have feelings for her? Though we feel for Marianne after the way he uses her, we can’t help but wonder.
Willoughby seems genuinely uncomfortable when he tries to talk to the Dashwoods about why he is going to London.
"It is folly to linger in this manner. I will not torment myself any longer by remaining among friends whose society it is impossible for me now to enjoy." (ch 15)
They have no idea what he is talking about, and begin to doubt his intentions. Willoughby seems torn between his heart and his obligations. While not intentionally leading Marianne on, that is effectively what he is doing. He may not be trying to be cruel, but the end result is the same.
When Marianne addresses him at the dance, where he is in the company of a fashionable young lady, he seems genuinely embarrassed. It is as if he has no idea what to do, and how to react. He tries to pretend he does not know what she is talking about, because he does not want to face the reality of the situation.
Despite what he has done to her sister, Elinor does seem to believe Willoughby when he tries to explain and apologize with “serious energy.”
I mean to offer some kind of explanation, some kind of apology, for the past; to open my whole heart to you, and by convincing you, that though I have been always a blockhead, I have not been always a rascal, to obtain something like forgiveness …. (ch 44)
Ultimately, Colonel Brandon makes a little bit more stable choice of husband for poor Marianne. Neither she nor Willoughby were being sensible in their romantic notions. She was a poor girl, left penniless by the vagaries of English property succession laws. He could never marry her, even if he still loved her. Regardless of his intentions, he still hurt her.