Jack is the antagonis(competitor, rival) of the story
"Antagonis" appears to be a misspelling or variation of "antagonist." An antagonist is typically a character or force in a story that opposes the protagonist, creating conflict. In a broader context, it can also refer to any opposing force in various fields, such as biology, where it describes a substance that interferes with or inhibits the action of another.
I Gede Rai Maya Temaja has written: 'Pemanfaatan mikroba antagonis sebagai agen pengendalian penyakit akar putih tanaman jambu mete'
In the story "The Aged Mother," the protagonist is the aged mother and her son. The antagonist is the cruel ruler who orders all old people to be taken to mountain to die, including the aged mother.
The Agonist muscle. In contrast, the muscle group that has passive action(the muscle being stretched) is called the antagonis muscle. For example, when doing a bicep curl. Your biceps are the agonists and your triceps would be the antagonists.