He might be, he might not be. Why spoil the story? He is not a playable character if that's what you are asking. The Lost Legacy is a spin off that lets you play as Chloe from the second Uncharted game. You do not play as Nathan Drake. Whether he is in the game anyway, there is no point in saying either way. The game is shorter than past games so you will find out when you play the game.
