Percy is John's in to a world of luxury, ease, and garish wealth. Though we don't know it at the start, we find out later that Percy willfully brought John to his home to die; he knows full well that his father murders all his family guests. Percy drops out of the story once Kismine enters the picture, so we never get to hear his thoughts on the matter or listen to his attempts at justifying this behavior. We assume, though, that he shares his sister's shallow feelings on the situation. John does conclude, after all, that both "Percy and Kismine seemed to have inherited the arrogant attitude in all its harsh magnificence from their father," and that "a chaste and consistent selfishness [runs] like a pattern through their every idea" (8.2).
Percy takes on another important role when he fills John in as to the history of the Washington family. This is the part of the text where we start to see the connection between the history of the Washington family and the history of the US. This is where the satire aspect of the story kicks in (see "Symbols, Imagery, Allegory" for a discussion of how this all works). As the hidden narrator of his family's history, Percy provides John – and the reader – with all the background info we need.
We don't get to see anything of Percy in the dramatic bombing scene, but we are told that he goes into the mountain to die with his family. We're not sure how Percy feels about this whole thing, or what his reaction is to the attack. He seems to be fated, however, like his father, to go down with the ship.