Naiad

The naiads are creatures that live in Poseidon's realm the sea. Charybdis, one of the monsters in the Sea of Monsters, was once a beautiful naiad as well, although she was turned into a monster because Zeus was angry that she was taking so much land. Many live in the Canoe Lake at Camp Half-Blood.

Their cousins, Nereids, are spirits of the sea. They serve in Lord Poseidon 's court. Nereids also carry white pearls which give the hero a quick escape to the underworld. Also when Nereids leave, they glow green and vanish. When Percy was young, Nereids would watch him with interest.

The Lightning Thief
In The Lightning Thief, Percy meets a Nereid though he didn't know what she was at the time at The Arch when he battled Chimera. The Chimera poisoned Percy, so he jumped off the Arch in St. Louis in the Mississippi River, which saved his life. Then he sees her and she says, "Go to the beach in Santa Monica." He does and meets her again. There, Percy can see her better too. The Nereid gives him three white pearls and leaves him for the last time.

The Titan's Curse
While Percy and Zoë were in the canoe together, one spit water at her -- she said that they had never forgiven her, maybe because she had become a Huntress instead of staying with the Hesperides.

The Battle of the Labyrinth
There was a naiad that had helped Percy to try to clean the flesh-eating horses' stables. It reminded Percy of Rachel Elizabeth Dare, because she was asking so much questions; Percy felt like she was throwing rocks at him.