]Willoughby has figured out Marianne's penchant for the romantic, the passionate and having similar minds with whoever she plans to be in love with and he knows how to make himself fit the mold. He manages very well to keep himself the apple of Marianne's eye while making sure the Colonel is relegated to nothing but a core which we all know is tossed away.

Good metaphor!

I also just thought of this: I think Willoughby is enjoying the power he is exerting over Marianne at this time, seeing the effects of his charm at work, so to speak. He's the favoured male in the group, and it must be gratifying to his ego to hear Marianne agree with him and willing to call down or criticize anyone Willoughby does. Every time he says something against Brandon (or Mrs. Jennings or the Middletons) he can be sure that Marianne will pipe up in agreement, and it's like a stroke to his ego or confirmation of his own supremecy with her.