User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-5774152-20130221034225/@comment-2079715-20130302092436

This text wall has even more convinced me that it is Nico who will stay behind. Percy will have to make sacrifice he cannot make. It will have to be a person he cares about a lot. And of all the characters available in Europe, Percy knows best Annabeth, Nico, Frank, Hazel, Jason, Leo, Piper, Coach Hedge. In that order. So, i truly believe Nico will be the great Percy's sacrifice, and Frank will have to help him do it.

Percy and Annabeth had the pinnacle of their love in Mark of Athena, as Darkcloud said. Percy isn't staying behind.