Okay, how to do this with out ALL THE SPOILERS? Hmm.
Let's put it like this; Grassblades is in large part about the journey its protagonist, Masahiro, makes - and most of the time he's fighting against himself, because he fell out of the Oh-god-do-I-ever-have-issues-tree and hit every branch on the way down. BUT there are definitely external antagonists in the story.
The approach I'm taking is three-fold, because I've got three types of antagonists I'm working with.
1.) The people with opposing goals. Masahiro is travelling somewhere, and he's travelling for a reason - he wants to go on doing it until he reaches his goal. I've got another set of reoccurring characters (who will be introduced in this current chapter!) who will try to stop him from doing so; they have their own reasons for doing so, none of which are cruel or evil. It's just the task they've been set to do. But as they are opposing the goals of the protagonist(s), so they are, in function at least, antagonists.
2.) The people with bad intentions. These lot are more true antagonists; they will be the people who, once they show up in the story, will establish their will to not only hinder Masahiro, but to actively hurt him. They're the stab-people-in-the-back-for-fun-and-profit type, though you can switch out "fun" for "furthering their own goals".
3.) The scary demons. Exactly what it says on the tin; while it hasn't been immediately obvious, Grassblades is a fantasy-comic. For more than one reason, Masahiro is going to be forced to use his +1 sword of stabbity on more than one supernatural beastie.
From this list, it should be clear that I prefer antagonists who are people (when, uh, they are actual people, and not supernatural beasties, of course). I have about 0% interest in antagonists who fall into the cackling mad evil overlord category, because there is no tension in that. They're just cackling mad and evil. Conflict becomes a lot more potent when you can't just point to the antagonist and say "Well, we're fighting them because LOOK HOW EVIL THEY ARE!" There's got to be more to it than that. I might not what my readers to sympathise with all of my antagonists (except the first group), but I do want them to emphathise with them. You don't need to be cheering them on, but you do need to understand why they do what they do.