The two main characters I'd point to are Tabitha, the apprentice gatekeeper, and Silas, the morally ambiguous traveler she reluctantly lets through. Their dynamic is the engine of the story—he’s all sharp edges and hidden motives, and she’s trying to hold onto her principles while learning the world isn't as black and white as the Gatekeeper's Code made it seem.
What I found really interesting, though, were the secondary characters that orbit them. There's the enigmatic Master Kael, who I suspect knows way more than he lets on, and Lira, the herbalist in the nearby village who becomes Tabitha's only real friend. The antagonist isn't just one person either; it's more this creeping sense of institutional rot within the Gatekeeper Order itself. You start questioning who the real 'gatekeepers' of truth are by the end.
My reading group actually got into a huge debate about whether Silas qualifies as a protagonist or just a catalyst. I'm in the 'catalyst' camp—the story is fundamentally about Tabitha's transformation, and he's the wrench thrown into her carefully ordered life.