One of the big themes in Blue Lily, Lily Blue was that of the three sleepers along the ley line. Of course, the third sleeper, the one not to be woken, proved to be a demon; the middle sleeper was arguably Gwenllian, though in the closing chapters of Blue Lily Adam wonders if it might have been himself, in a metaphorical sense.
But what about the first sleeper?
After all, the characters assumed that it was Glendower all through the series, but The Raven King revealed that Glendower was dead, not sleeping, all along. So who was the first sleeper?
We were given the answer all the way back in Blue Lily, Lily Blue chapter 15. While communicating with Cabeswater, Adam draws three tarot cards, each representing one of the three sleepers: the Devil, for the demon, the Empress, for Gwenllian, and Death. What else—who else—does the Death card represent in this series?
Gansey. All the way back in the reading in The Raven Boys, he drew it as the card to represent himself. And after all, all times are equal on the ley line, so theoretically, Gansey was always sleeping on the ley line, throughout the entire series. And if you need more convincing:
Calla tells Blue that it’s her job, specifically, to wake one of the sleepers (BLLB ch17). And what does Blue do at the end of The Raven King?
Monsters' Queen 