Antagonists are an important part of any story, but they aren't always in the role that you may expect. The classic view seems to be that the antagonist is the villain to the protagonistic hero. However, the antagonist can be anywhere in a story, even on the hero's side. Let's talk a bit about that today.
Definition of an Antagonist
The definition of an Antagonist that I've always liked comes from John Wick. Essentially it goes like this: over the course of the story the Antagonist forces the Protagonist to change. In other words, whomever is forcing change to happen is the/an antagonist. Usually this is the villain as they are the ones setting things into motion to cause change in the hero. This could be Darth Vader in Star Wars whose actions force Luke onto a heroic journey and then a final confrontation with Vader and the Emperor. However, it can also be something else too.
For example, in a lot of ways Obi Wan Kenobi and Yoda are also antagonists. Despite being on Luke's side they do force Luke to change and grow to the situation. Breaking away from Star Wars though, we can find a great example of a non-villain antagonist in the movie DREDD.
In Dredd Karl Urbin's DREDD is more of an antagonist and Anderson is more of a protagonist. I say more of because in the end Anderson does make Dredd change a little bit, but that change is only in recognition of the change Dredd forces on Anderson. Throughout the movie Dredd is pushing Anderson, challenging her, and forcing her into trials.
But how do you use this in game?
The Mentor Antagonist NPC
This is probably the easiest antagonist NPC to put in your game that isn't a villain. However, for this to work you need to have a PC who needs a mentor, and an NPC who can be that mentor. From there you make the NPC rigid and unchanging, but preferably an ideal that the PC can strive for. From there you just need to challenge the PC. Call them to task when they stray from that ideal. Show them when they've grown closer. Don't force either or to happen, but react when it does happen.
The important thing with the mentor is that you don't make them a boss. Mentors are meant to guide and show, not force, someone down the way. The choices need to be made by the player. The mentor can be disappointed or elated with the PC's choices, and that can be part of the reward/reaction to a choice, but try not to influence the choice.
The Antagonist As Quest Giver
This one is one of my personal favorites when it can be done, because it lets you hide an antagonist right in the middle of the PCs. You can use the quests you give to your players to make them change. How the PCs go about the missions given to them, how they interact with NPCs, and what they confront on those missions are all things that you can control and that can prompt the players to change. Put an NPC as a central quest giver, giving the PCs quests and missions to show them a certain side of the world or a small part of it can force them to change. When the NPC does that, they become an antagonist.
Where else have you managed to put an Antagonist? Where did you get the best results? The fun part about moving them around is that when the PCs don't realize they're being prompted to change, sometimes they grow in the most awesome of ways.