The character that opposes the main character is called the
antagonist
static character
protagonist
foil character
flat character

Antagonist

Explanation: The character that opposes the main character is called the Antagonist. As it suggests an antonym to the protagonist of the story. The Antagonist is the one who not merely opposes the actions of the protagonist, but also plays a chief role in the development of the character throughout the story and also responsible for the course of action in the story and evolution of the protagonist to gain sympathy of the readers at the end. Thus, we may conclude that an antagonist is as essential as the protagonist otherwise, the work would become so monotonous and the readers may lose their interest in the work.