Cady is the daughter of Mimi, and an ex-girlfriend of Sam Oliver and is briefly suspected to be the daughter of the Devil, whom Mimi had a twenty-year-long "relationship" with. Sam's evidence for her being The Devil's daughter included flowers dying in her presence, a fish going belly-up after she chose it for her next pet, and potentially Demonic snoring. Sock decided that checking for the mark of the Devil (666, like in The Omen, according to Sock) would be sufficient evidence, which of course meant: seeing her naked.
Cady was surprisingly cool with this.
And as it turned out, she had no weird birthmarks. Sam asked The Devil straight-out and The Devil said she wasn't, and even let Sam check her out with some glasses that supposedly would reveal The Truth ("Unseen"). But by the time Sam was totally satisfied, Cady was fed-up with his weird behavior anyway and left, to follow her mom to New Mexico. She hasn't been seen since.
It's been speculated that most of that weird stuff that happened around her was because of Sam, and that maybe he shoulda looked in a mirror while wearing those glasses...