We have discussed already that every good story puts the characters in front of some sort of conflict. And nothing is better than the personification of the conflict itself: the bad guys! Villains, monsters, and enemies.
Despite the fact that all these guys intend to kill the player, they serve the higher purpose of letting the player having fun (while he avoids being killed).
But the question is, how are the bad guys designed?
The answer, as often happens in game design, is not a simple one and depends on the type of game we’re talking about. Simple enemies, in the same way as the other characters, exist to exert a function. Villains and Bosses do that too, but they also need to be memorable and as interesting as the main characters. Games with deeper narratives rely on well-constructed villains to oppose the protagonist, while more arcade and less narrative heavy games need enemies that offer a challenging focus on the gameplay.