Percy Jackson is not just an alpha, he is the alpha. The savior of Olympus, the leader of the seven to fight the Giants. Nico is from the forties, where alpha-alpha couples had been a huge no-go. So as an alpha, he knows he has no chance with Percy. Percy all the while is ashamed for being an alpha who wants to submit, something alphas shouldn't want to. Nico/Percy slash Nicercy