Talk about transformation. At the beginning of the book, Aunt Casey is "tall and slim and stiff-legged in her too-high heels" (1.21), and according to Miracle, resembles a Barbie doll. She's spiteful, nasty and "always had something mean to say about Dane, or me, or both of us" (1.20). As a result, we can't stand her any more than Miracle can. But before you write Aunt Casey off completely, consider this: Next to Miracle herself, Casey ultimately undergoes the greatest change of any character in the book, so by the end of the story, we not only like her, we're rooting for her and Miracle to rock their new life together.
The change in Casey's character starts around the time when Miracle comes to live with her and Toole, and she decides to go for a degree in psychology. Miracle's first clue that something's up is how she looks:
She had even begun to dress differently. Instead of spandex and tight sparkly shirts, she wore baggy jeans and extra large tee shirts […] Her hair was combed down, too, less stiff, and she kept it dyed red and wore almost no makeup. (14.23)
Unless she's the human model for Collegiate Barbie, Casey's definitely toned things down a ton. But her changes aren't just superficial, and Casey also changes her tune when it comes to Miracle. While we're certainly not justifying Casey's behavior at the beginning of the book, we can kind of understand why, given Sissy's suicide, she has a hard time seeing Miracle for the person she is instead of just the source of so much of her sister's misery.
Where she was once overwhelmed with guilt and hard feelings, though, in working with Miracle at The Cedars and walking her niece through the truth about her sister's death, Casey comes to recognize that things don't have to be this way for the two of them. "I see now," she tells Miracle, "I know now. You're not my punishment, you're my opportunity" (27.26). And when she says this, we don't doubt that Casey is officially on Team Miracle for life.
While we may not think too much of Casey from the beginning, by the end, she's become a strong woman committed to living with Miracle in the real world rather than the family's fantasy past. Plus, she doesn't wear tacky clothes anymore, so victory all around.