Jack is the antagonis(competitor, rival) of the story
I Gede Rai Maya Temaja has written: 'Pemanfaatan mikroba antagonis sebagai agen pengendalian penyakit akar putih tanaman jambu mete'
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.