Antagonist

Antagonist
An antagonist is a person or a group of people who oppose the protagonist's opinion or actions in a story. The antagonist is the "bad guy " of the story, he dislikes, disapproves of, or opposes the ideas, actions, or opinion of the protagonist and even acts against him.

Example
In the Lord of the Flies, if Ralph is the main character, Jack would be the antagonist. You could even say that there are several antagonists, the entire group of hunters could be a group of antagonists.

Quote from the book
"He laid the conch with great care in the grass at his feet. The humiliating tears were running from the comer of each eye. “I’m not going to play any longer. Not with you.” Most of the boys were looking down now, at the grass or their feet. Jack cleared his throat again. “I’m not going to be part of Ralph’s lot-” He looked along the right-hand logs, numbering the hunters that had been a choir. “I’m going off by myself. He can catch his own pigs. Anyone who wants to hunt when I do can come too.” He blundered out of the triangle toward the drop to the white sand. “Jack!” Jack turned and looked back at Ralph. For a moment he paused and then cried out, high-pitched, enraged. “No!” He leapt down from the platform and ran along the beach, paying no heed to the steady fall of his tears; and until he dived into the forest Ralph watched him. Piggy was indignant." Taken from page 183 the online book.