The whole of Lucy's behaviour in the affair, and the prosperity which crowned it, therefore, may be held forth as a most encouraging instance of what an earnest, an unceasing attention to self-interest, however its progress may be apparently obstructed, will do in securing every advantage of fortune, with no other sacrifice than that of time and conscience. (Ch. 50)

Now that Lucy has revealed herself completely she seems to be a female version of Willoughby. I think the above description of Lucy would do for him as well. They are both users. They manipulate others for their own ends and obviously consider other people’s feelings and welfare insignificant compared to their own. Lucy would have fixed her hopes on Mrs. Ferrars death to free Edward to marry her if there was an end in sight per Ch. 24. Lucy and Willoughby are both fortune hunters, determined to make their fortune by marriage even if the cost is a loveless one and they easily dispose of people who are no longer of use to them. Lucy enters into a secret engagement and if one was suspicious they might note the physical evidence of Edward’s commitment in letters and gifts so very useful in proving her engagement to Elinor. I imagine Willoughby’s courtship of Sophia is similar to Lucy’s in that he needed legal commitment to obtain her fortune but his dealings with other women are just the opposite. He deftly avoids legal entanglements so that he can discard girls when they no longer fulfill his needs. Neither Lucy nor Willoughby buck at ignoring moral obligations they have incurred yet seem to depend on their victims acting with honor towards them. Lucy depended on Edward keeping his promise to her out of honor while Willoughby works under the cover of honor and respectability. No one suspected his lack of honorable intent towards Marianne.

The fact Lucy’s “letters to the very last were neither less frequent, nor less affectionate than usual” (Ch. 49) could mean she had some affection for Edward in the beginning and kept up a pretense when her affection faded or perhaps she never cared for Edward other than as a meal ticket and her affection from beginning to end was pretense. Lucy says “Being very sure I have long lost your affections, I have thought myself at liberty to bestow my own on another” and it suggests two ideas to me. First, the implied easiness of a transfer of her affections, if they actually existed, show they were not strong and like Willoughby’s for Marianne easily gave way to self-interest. The fact Lucy kept Edward tied to her despite the conviction he had grown weary of the engagement and the risks he bore for the connection illustrates she was not concerned about his happiness or best interest just as Willoughby only considered his interests in his dealings with Eliza, Marianne, Sophia and nameless others.

There are probably more to be found but here are a few other similarities between Lucy and Willoughby. Both confess to Elinor, in part, with the hope she can further their goals. Lucy wanted Elinor’s help influencing her brother to give Edward the Norland living (Ch. 24) and Willoughby wanted Elinor to pass his confession to Marianne to be in her good graces again (Ch. 44). While confessing they both throw blame on others in an attempt to gentle their guilt:

"I was very unwilling to enter into it, as you may imagine, without the knowledge and approbation of his mother; but I was too young and loved him too well to be so prudent as I ought to have been. -- Though you do not know him so well as me, Miss Dashwood, you must have seen enough of him to be sensible he is very capable of making a woman sincerely attached to him." (Ch. 22)

I do not mean to justify myself, but at the same time cannot leave you to suppose that I have nothing to urge, -- that because she was injured she was irreproachable; and because I was a libertine, she must be a saint. If the violence of her passions, the weakness of her understanding -- I do not mean, however, to defend myself." (Ch. 44)

They both have a way of casting their bad behavior as instruments of compassion for themselves:

I only wonder that I am alive after what I have suffered for Edward's sake these last four years. Everything in such suspense and uncertainty, and seeing him so seldom -- we can hardly meet above twice a-year. I am sure I wonder my heart is not quite broke." Here she took out her handkerchief; but Elinor did not feel very compassionate. (Ch. 22)

But I thought of her, I believe, every moment of the day. If you can pity me, Miss Dashwood, pity my situation as it was then. With my head and heart full of your sister, I was forced to play the happy lover to another woman! -- Those three or four weeks were worse than all. (Ch. 44)

Willoughby eventually inspired compassion and forgiveness from Elinor although later qualified in Ch. 45 as “a tenderness, a regret, rather in proportion … to his wishes than to his merits” due to his “person of uncommon attraction -- that open, affectionate, and lively manner which it was no merit to possess and by that still ardent love for Marianne” whereas Lucy never inspires feelings beyond a little pity every now and then. For example, to Lucy’s utter amazement Elinor “assured her, and with great sincerity, that she did pity her” in Ch. 34 when they were to meet Mrs. Ferrars. I think there are two reasons for this. Elinor’s compassion is stirred by Willoughby because she believed he had affection for Marianne whereas I do not think she was convinced Lucy ever had any for Edward. Additionally, as Elinor realized in Ch. 45, Willoughby’s natural ability to gain peoples good opinion and affection are rather extraordinary and I think Lucy’s often depend on the failings of other people’s understanding. Lucy has to work harder. She is finally “openly acknowledged, to be a favourite child” by Mrs. Ferrars after her “perseverance in humility of conduct, and messages, in self-condemnation for Robert's offence, and gratitude for the unkindness she was treated with” in Ch. 50. Mrs. Ferrars is won over by Lucy’s insincerity and servility. Willoughby could fool many people much of the time (except Col Brandon) but Elinor pegs Lucy early on in their acquaintance:

Elinor saw …with less tenderness of feeling, the thorough want of delicacy, of rectitude, and integrity of mind, which her [Lucy’s] attentions, her assiduities, her flatteries at the Park betrayed; and she [Elinor] could have no lasting satisfaction in the company of a person who joined insincerity with ignorance; whose want of instruction prevented their meeting in conversation on terms of equality, and whose conduct towards others, made every shew of attention and deference towards herself perfectly valueless. (Ch. 22)