Just Who Can They Be?
The conflict between a hero and villain is a common core of a good story plot. It's not essential to have a villain, of course, for the main character, the protagonist, might face other kinds of conflict in a good story. If you've ever taken a literature class, you might find struggles against nature, society, or even the hero against himself as a source of conflict. But, more often than not, especially in fantasy, there's a good villain determined to take over the world and an equally good hero determined to stop him.
Read or watch any superhero story and both hero and villain are clearly defined. The hero is a master of virtue, with superhuman powers he/she uses for the good of mankind. The villain might be equally as powerful with clearly evil, destructive intent and very few, if any redeeming qualities. Good and evil a clearly defined and there's never much of a question that it's the hero's duty to make things right.
None of that is quite the same as real life where people's motivations are not always so clear cut. While fantasy authors do want readers to be able to step into their novels and out of the real world, it's important to keep some of the truth of the real world intact in the fantasy. Creating characters, both good and evil, the reader can relate to on a human level is a good part of that.
It helps if characters have emotions readers can relate to--love, hate, anger, joy, fear, confusion. They need to have challenges to overcome and the skills to overcome them without being totally invulnerable. Even Superman has his Kryptonite. In classic literature, the tragic hero always has a fatal flaw. We want our heroes to survive the story, but not without some trouble along the way.
And the villains? Well they need their fatal flaws as well. Sometimes it's just their own overwhelming belief that they really are unconquerable. The villain's evil nature somehow has to balance out the hero's need to destroy him. The hero's stature is diminished if the reader isn't quite convinced the villain is really worthy of meeting his end at the hero's hand. Unless the story is intended to blur the lines of good and evil to pose moral issues, most villains shouldn't really earn too much admiration as the story progresses.
Creating effective characters on both sides of morality is a tricky balancing act. Writers need to be keen observers of the real world and learn all they can about real people. Learning what motivates people, what moves people, and what drives people to choices and actions is a skill authors need to develop. Only then by using that knowledge to develop effective characters can they bring a fantasy world to life.