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 immortal Gorgon sisters of Medusa and Stheno. Despite having faded earlier, Gaea revived her at some point.

The Lightning Thief
While not seen in the book, Medusa tells Percy Jackson, Annabeth Chase, and Grover Underwood 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.

During an assault on Camp Jupiter, Euryale and her sister attack the giant eagles. They are both killed at the end of the battle, possibly by Mrs. O'Leary.

Personality
Euryale is far more narrow minded than Stheno and crueler. She appears to be the dominant sister as Stheno seemed nervous when she began to shout for not following her plans, and she always orders Stheno around. Euryale is also smarter than Stheno but not more than Medusa.

Trivia

 * According to Medusa in The Lightning Thief, Euryale and Stheno had both faded yet they appear in The Son of Neptune, meaning Gaea may have some way to bring back even faded beings.