Harpies (meaning "snatcher") are winged spirits of sharp wind. They are known as the hounds of Zeus. Sudden, mysterious disappearances are often attributed to harpies.
The harpies were once sent by Zeus to plague King Phineas of Thrake (a son of Neptune) as punishment for revealing the secrets of the gods through prophecies. Whenever a plate of food was set before him, the Harpies would swoop down and snatch it away, befouling any scraps left behind. When the Argonauts came to visit, the winged Boreads Calais and Zethes gave chase and pursued the Harpies to the Strophades Islands, where their sister Iris commanded them to turn back and leave the storm-spirits unharmed.
At Camp Half-Blood, there are "cleaning harpies" named Aello, Celaeno, and Ocypete. These harpies that come to eat demigods that sneak out of their cabins at night. They always refer to themselves in the third person, for example they screech; "Campers out of bed! Dinner for hungry harpies!" Percy Jackson refers to the cleaning harpies as if they were "crossbred with the dodo birds." They wash the dishes with lava which, according to him should kill 99.9% of all germs.
It is revealed that some harpies work for Aeolus. This is likely because they are spirits of the wind. They are mentioned to be employed by Aeolus and create a bridge for Jason Grace, Leo Valdez, Piper McLean, and Gleeson Hedge to walk across.
Phineas was revived by Gaea, he is now the one to eat the food and be able to keep the harpies from his table though they are bounded to eat and can't consume any other food.
Ella is one of the many harpies who snatch food from Phineas' picnic table. She's faster and more nimble than her sisters. When befriending with Percy, Hazel Levesque and Frank Zhang, she shows her ability to memorize a massive amount of facts she reads in books. Percy decides that they have to free the harpies from their curse (to be only able to eat from Phineas' table) which they accomplish with a gamble to the death, including Percy and Phineas both drinking Gorgon Blood, not knowing which vial was from which side of the monster. Later on, Ella is taken to Camp Jupiter by Tyson. When Percy, Hazel and Frank are talking about their encounter with Ella, they decide that she is too important to leave behind when they go to Rome because she has memorized a part of the Sibylline Books.
- Aerokinesis: Harpies can control and manipulate the air. They create sudden, sharp gusts of wind, capable to snatch away people and things from the earth.
- Flight: Harpies are able to fly through her wings.
- Fire Immunity: Like Cyclops, harpies are immune to lava and fire.
- The literal meaning of the "Harpies" seems to be "that which snatches."
- Some seem to be named after food products, as two harpies are called 'Nuggets' and 'Teriyaki' in The Lost Hero.
- The American Harpy Eagle is a bird named after them.
- Ella has said that cinnamon is good for harpies.