More often, stories simply do not have characters that are readily identifiable as most heroic or villainous. Instead, the antagonist becomes that character, group, or sometimes force which provides the chief obstruction to the protagonist or "main character" of the story. Note that the antagonist is not necessarily human; often, the forces of nature or psychological elements provide this element of opposition.
The protagonist-antagonist relationship is also sometimes ambiguous. For instance, in the story of Moby Dick by Herman Melville, the antagonist may be regarded as the whale "Moby Dick" of the title, against which the story's leading character Captain Ahab strives. Yet Captan Ahab is not actually the protagonist of the story, as it is told from the point of view of the narrator Ishmael. Indeed, it is also valid to look as Captain Ahab as the antagonist, with his fanaticism the force with which protagonist Ishmael must cope.