In any story, the main character is the protagonist, the central person which the whole story revolves around. He/she remains the central focus, the axis around which the other characters and plot will revolve.
And in opposition to this main character, is the antagonist. An antagonist is the character who acts in opposition to the main character, and the central character has to overcome this character to get to his aim.
Thus, the character who is the main character's opposition is the antagonist.
