Nico appears in the beginning of the book, He is visiting his half-sister Hazel, who was showing Percy (after losing his memory) Camp Jupiter at the time. Nico lies to Percy about knowing him after he finds out that Percy's memories have been stolen and simply says that he's an ambassador for Pluto at the Roman camp. It is later made clear that this was to preserve diplomacy (since Greeks and Romans didn't get along) and not interfere with Hera's plans - rather than a malicious urge.