What is the importance of the "white cat" in the novel, White Cat?

A white cat makes several appearances in this story. First it visits Cassel in a dream where it seems to bite off his tongue. He sleepwalks onto the roof while this is happening. Then it appears to him in another dream and tells him that he has cursed it, and that only he can remove the curse. Later an actual white cat visits him at his mom's old house and he brings it inside to look after it. After breaking into Barron's house he sees footage of his sleepwalking on the roof of the school and notices a cat in the background. In Barron's diary he sees a note saying that 'L is clearly there.'

Cassel begins to suspect that the white cat is really Lila and that he did not kill her after all. However, Philip has sent the white cat to an animal shelter. Cassel pulls a complicated con to get her out, and then tries to transform her back into Lila. He succeeds. The white cat has indeed been Lila all along. Even in cat form she can still manipulate dreams, so Barron and Philip have been using her to make people sleepwalk to them. Lila was turned into a cat when Cassel refused to kill her and transformed her instead. Barron has been keeping her locked in a cage in his house until she managed to get away. That was when she visited Cassel and gave him his first dream of the white cat.