Technically main characters are "protagonists" and opposing characters/bad guys are "antagonists". However, outside of my AP Lit class I've rarely called a main character a "protagonist" but "antagonist" is a word I can't help using. Why? Because not all opposing characters are actually bad. They're not all villains. But this is common knowledge, I'm sure, so I won't cite literary examples or anything. I'm sure you can all think of antagonists who weren't actually bad.
I'm a huge fan of bad guys who aren't pure evil--who have good sides. Mainly this is because unless your bad guys is the devil, Sauron, or some equivalent, he probably wasn't born evil. At some point in his life, he was probably a decent fellow--if not good--and he made some bad choices, like Saruman the White Wizard in LOTR. And then there are antagonists who aren't bad at all--they just stand in the main character's way.
Spirit Riddled has two kinds of antagonists. Those who are genuinely bad (but weren't born that way) and one who is actually good but just stands in Jess's way. How did I think of these antagonists? Well, it's all about creating conflict--something we still have a need to discuss. In the world of Spirit Riddled our three (that's right, three) antagonists came directly from the world building. We have the High Priest--who staunchly believes that all people with magic are demon-possessed. We have the Godfather like ruler of the thieves--who seeks to take out his revenge on Mage. And we have the ruler of the nation--who simply fears that Mage might overthrow him with all of his "magical" power. Basically, all my antagonists run on fear of these magical people.
Spirit Riddled is very much a story about fear in that way.
To create a good antagonist, you need someone who is going to oppose your main character, someone whose intentional and unintentional actions create road blocks.
Generally, my antagonists come in two sorts: products of the world and products of the main character. The products of the world are rulers, priests, lords--people who come form the world building, who would exist regardless of whether the main character existed. The products of the main character are people who are directly connected to the main character and who are unimportant otherwise: an abusive father, a jealous brother, a former best friend, a neighbor.
It's hard to talk about antagonists without talking about conflict, but fear not. Soon we will. And conflict is what drives the story.