Theo was mentioned briefly in conversation in the novella Stalked (featured in My Big Fat Supernatural Honeymoon) he is the father of the main antagonist in that story. He is the patriarch of the Cain family, as seen in one of Kelley's online stories. He is the father of Zachary Cain, who as seen as one of his favorite sons. He is the grandfather of Nate Cain, Carter Cain and Derek Souza.
He has dark hair and green eyes. He is smarter then the other Cains, but that isn't saying much. Derek describes him as "...huge, at least my height, with broad shoulders, graying dark hair and broad bull-dog face."