What advice did Nereid give Percy in the Jackson book?

Nereid is not a character in the Percy Jackson series. It seems you might be thinking of Nereus, the Old Man of the Sea, who gives Percy some advice in The Sea of Monsters (the second book).

Here is the advice Nereus gives Percy:

"You must be clever, demigod. Not strong. You must use your wits, not your fists."

Nereus warns Percy about the dangers of relying on his brute strength and encourages him to use his intelligence and cunning to overcome challenges. This advice proves crucial to Percy's success throughout the book.

Let me know if you have any other questions about the Percy Jackson series!