An antagonist is a character(s), institute, or concept that is used as a force of
opposition towards the
protagonist. Often, in most cases, the antagonist is a bad guy, a villain, however that isn't always the case. In some cases, the antagonist could very well be
a good person, a hero.
Person 1: Those cops in those movies where they try to stop criminals are good antagonists.
Person 2: But they are good people. How are they antagonists?
Person 1: Antagonists aren't
necessarily bad people. They just oppose the protagonists, in which some cases the protagonists is
the evil one. Like Death note. Light Yagami may be the protagonist, but you know he's
the evil one and L, the guy trying to stop him, despite him being the good guy, is the antagonist of the story.