Euryale

"Stheno, the Bargain Mart was a front! You’re going native! Now, put down that ridiculous tray and help me kill this demigod."

- Euryale yelling at Stheno

Euryale is one of the Gorgon sisters of Medusa and Stheno. Despite having faded earlier, Gaea revived her at some point. She appears in The Son of Neptune along with her sister Stheno.

The Lightning Thief
While not scene in the book, Medusa tells Percy, Annabeth, and Grover that her two sisters had helped her in her business, but had passed on.

The Son of Neptune
Euryale, along with her sister Stheno, had been repeatedly killed by Percy Jackson, but they just kept on reforming because Thanatos was unable to prevent monsters from coming back from the dead. She was determined to take revenge on Percy Jackson for vaporizing Medusa, who hadn't reformed yet.. She scolds her sister for telling Percy the secret of Gorgon blood and for being too obsessed with her work, her hair is all coral snakes.

Personality
Euryale is far more narrow minded than Stheno and slightly crueler. She appears to be the dominant sister as Stheno seemed nervous when she began to shout about not following her plans.