The school's strict Pokemon battle coach could be an antagonist. He might have old - fashioned ideas about battling and not approve of Ash's unorthodox methods. He could try to prevent Ash from competing in important school battles or even expel him if he doesn't conform.
Gary could be a main antagonist. He might be jealous of Ash's popularity at school and constantly try to undermine him in Pokemon battles and in front of their classmates.
A new student who has a very powerful and rare Pokemon could be the antagonist. This student might be arrogant and look down on Ash and his friends, and constantly challenge them to battles with unfair rules.