Man, I've seen a bunch of people tripping over this because 'Darkness' is such a common title—it's practically a genre of its own. If we're talking about the one that gets recommended a lot in fantasy circles, I think you mean the 'Darkness' series by someone like Harry Turtledove? Wait, no, that's not right. The confusion is real. There's a novel just called 'Darkness' by a guy named R. M. Lamsley? I haven't read that one. The most likely candidate people ask about is actually a book often paired with 'Dawn' and 'Day'—that's 'The Darkness' that's part of 'The Fifth Wave' series by Rick Yancey. In that case, the protagonist is Cassie Sullivan, a teenager trying to survive the alien invasion. She's tough, resourceful, and the whole story is from her perspective, dealing with loss and trust in a pretty messed-up world. But if it's a different 'Darkness', all bets are off. I remember borrowing a book with that title from the library once, something about a detective in Alaska, and the main character was this grizzled ex-cop. Titles really need to be more unique.
Honestly, without the author's name, pinning down 'the' protagonist is like trying to catch smoke. It could be a romance protagonist staring into the 'darkness' of their soul, or a horror protagonist being consumed by it. The question feels like it's missing a key piece. My advice? Next time someone brings it up, ask for a detail—the cover, a character name, anything. Otherwise, we're all just guessing in the dark, which is ironically fitting.