Up until now I thought that the narrator was Nemesis since her reveal during the Ruthless Route, but now that you mention it, it might be possible that there are multiple narrators, since there are times when they refer to themselves as a collective rather than a person. It would also line up with what Argos says about the Labyrinth having "multiple voices", though that might be completely unrelated phenomena.
Even so, I'm still inclined to believe that it's Nemesis who watches us during the whole story. There's some very explicit dialogue outside of the Ruthless Route that hints to her being the narrator (Ch. 17, after forgiving Nikos: "Oh, [name], what a fine art you make of this brand of justice! This ceremony of yours has been nothing less than the finest of hymns to our ears. Stay this course, and we may yet award you a fine pair of wings to match your fine-weighing scale."). The way she uses "we" is more likely a reference to her sisters.