As conflict is a key element of most stories and games, it is worth considering the role of whoever or whatever opposes to the protagonist. What are the defining characteristics of an antagonist?
The antagonist is an opposing force to a character. This may be a physical opposition but more often an opposition of ideology, philosophy or paradigm; in Gladiator, Maximus and Commodus are polar opposites; one a family man, battle-skilled general and highly principled while the other kills or imprisons his own family for power, has no experience of war outside training in personal combat and exemplifies decadence.
By the end of the story, the protagonist and antagonist need to have resolved a significant issue, which usually is a win for the protagonist. The root of interplay for protagonist and antagonist comes from the Greek play tradition of agon - where each side responds in an argument with the chorus as judge and the second person to speak traditionally wins for they have the last word.
An antagonist also embodies the internal conflict of the protagonist. In The Dark Knight and Batman comics, both the Joker and Two-Face show aspects of Batman; the former his outsider status and capacity for violence, the latter his dual identity and sense of vigilante justice - it's a measure of Batman's complexity as a character this actually works without seeming like an excuse to pile on villains for the sake of it.
If you prefer the Bard, Hamlet and Laertes are another example - the latter shares the former's passion for justice and obligation to family and the deaths of Polonius and Ophelia set these two on a collision course played out in the duel between them in the final act. The two tortured characters are mirror images of each other and it's this similarity that makes the inevitable tragedy poignant.
An antagonist must also prey on the weaknesses of a character, be it the protagonist or another character known to the protagonist. Lex Luthor makes an effective foil for Superman by access to kryptonite while Hannibal Lector exposes and exploits the psychological foibles of both Will Turner and Clarice Starling as well as his numerous patients. Having an inactive antagonist doesn't do very much for anyone.
There are many ways an antagonist can work to further the plot. Not all need to be villainous, Lieutenant Gerard from The Fugitive is one example of a noble antagonist - others exist where moral ambiguity exists in the plotline or the character needs to develop before facing off against a more threatening and definitely villainous antagonist or a mutually destructive situation.
A good antagonist has these traits and others that help make them admirable, if not sympathetic. They need to be given some depth to engage the audience and this means development and more than two-dimensional plotting of the character. It's said that the measure of an individual is by their enemies, ensure the protagonists are getting their money's worth and your audience will surely do the same!
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