Jack is Bod’s direct antagonist. He kills Bod’s family, and he wants to kill Bod too. You can’t get much more antagonistic than that. Jack is a member of the Jacks of All Trades, a mysterious and ancient brotherhood. As Bod learns in Chapter 7, for the Jacks to exist, Bod must die, according to an ancient prophecy. So, for Bod to live (or so it seems) all the Jacks must die or at least be stopped somehow. Although Bod has other antagonists in the novel – Maureen Quilling, the ghouls, Abanazer Bolger – it Jack’s antagonism that shapes Bod’s life, and follows him all throughout the book.