Stheno

Stheno is a Gorgon sister of Medusa and Euryale. She and her sister Euryale were immortal while the third sister, Medusa, was mortal. She had faded, but was revived by Gaea by some manner. She appears in The Son of Neptune.

The Lightning Thief
When Percy, Annabeth, and Grover arrive at Medusa's shop, Percy asks what happened to her sisters. She claims that she is the only one left, alluding to her sisters, the Gorgons.

Battle of the Labyrinth
On their quest through the Labyrinth ,Tyson urges Briares to summon his brothers to help the unprotected Camp Half Blood. Briares tells the demigods that his immortal brothers do not exist anymore. Percy then remembers that Medusa's sisters also were forgotten immortals and they passed on.

The Son of Neptune
Percy Jackson first saw her on a Bargain Mart store posing as a greeter. At first Percy thought that her name is "Beano" until he decoded a big green button that said, "Welcome! My name is Stheno." She is described by Percy as appearing like a dumpy old grandmother, but with rooster feet and bronze boar tusks sticking out of the corners of her mouth, and her hair is actually vipers. Percy already killed her but she, along with her sister Euryale, just kept reforming, possibly due to being in contact with the earth, Gaea's domain.

Personality
Stheno has a relatively carefree personality with a somewhat naive disposition.