No characters in this novel are perfect. Even the people we're meant to admire, like Rachel and Franklin Blake, have plenty of faults.
But then there's Miss Clack.
She's practically a caricature. She's so unlikeable that some readers complain that she's too unrealistic. What good qualities does she have to balance out the bad? Why would Wilkie Collins, who is generally so careful to make his characters seem believable, make one of the main narrators so one-sided?
We know that Miss Clack is a distant relative of Lady Verinder and Rachel, and that she's much poorer than they are. Penelope mentions to Gabriel Betteredge that Miss Clack generally "fasten[s] herself on" Lady Verinder whenever they visit London. From Penelope's point of view, it seems that Miss Clack is like a parasite: she "fastens on" to try and suck up to Lady Verinder, hoping for free meals and maybe some money. But is it possible that Miss Clack is really just an affectionate relative, wanting to spend time with her Aunt while she can? Let's take a look at a few passages from Miss Clack's narrative…
Little did my poor aunt imagine what a gush of devout thankfulness thrilled through me as she approached the close of her melancholy story. Here was a career of usefulness opened before me! Here was a beloved relative and perishing fellow-creature, on the eve of the great change, utterly unprepared; and led, providentially led, to reveal her situation to Me! (188.8.131.52)
After Lady Verinder tells Miss Clack that she is terminally ill, Miss Clack's response is secretly to rejoice: she claims, at least, that she's glad to have the opportunity to guide Lady Verinder in her last days. She wants to be able to "prepare" her "beloved relative" for death, so that Lady Verinder will go to heaven.
But just a page later, we're given a hint as to the real reason that Miss Clack might feel "thankfulness" at hearing that her Aunt, Lady Verinder, is going to die. Mr. Bruff, the lawyer, is there to draw up Lady Verinder's final will, and he tells Miss Clack that she can be a witness because she does not have "the slightest pecuniary interest in Lady Verinder's Will" (184.108.40.206). This means that Miss Clack isn't going to inherit any money from Lady Verinder. Miss Clack's response exposes her disappointment, although she tries to cover it up:
Not the slightest pecuniary interest in Lady Verinder's Will. Oh, how thankful I felt when I heard that! If my aunt, possessed of thousands, had remembered poor Me, to whom five pounds is an object—if my name had appeared in the Will, with a little comforting legacy attached to it—my enemies might have doubted the motive which had loaded me with the choicest treasures of my library, and had drawn upon my failing resources for the prodigal expenses of a cab. […] Much better as it was! Oh surely, surely, much better as it was! (220.127.116.11)
Miss Clack describes how "comforting" it would be if her aunt had left her some money, especially considering that her aunt is rich and could totally afford it! But then she insists that it's "much better as it was!" because this way, no one will think that she's only trying to spend time with Lady Verinder for selfish reasons. Sure, Miss Clack. Who on earth would think that? By repeating her insistence that it's "much better as it was," Miss Clack shows us that really, she thinks the opposite. And what do we call someone who thinks one thing, and says the opposite? That's right: a hypocrite.
This brings us back to the original question: why would Wilkie Collins include a character as despicable as Miss Clack, without giving her some good qualities? Well, one reason might be that she's there to provide comic relief. After all, it's fun to read between the lines of her narrative, like we just did in the passage above, and point out the places where Miss Clack is being the most hypocritical. And if Collins gave Miss Clack good qualities – say, if she were really a generous person, and really did feel sorry for her aunt – we might end up feeling sympathy for Miss Clack. After all, she's lonely and poor, and has very few friends. But feeling sympathy for her would spoil the fun!