Stheno

Stheno is one of the Gorgon sisters 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 along with her sister Euryale.

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 and her hair is actually vipers. She also has bat wings on her back and tusks sticking out the corners of her mouth. She is said to always carry around her big silver platter of free samples: Crispy Cheese 'n' Wieners. Percy already killed her but she, along with her sister Euryale, just kept reforming, due to Thanatos being chained. To get away from them, Percy uses the plate of free samples to snowboard down the hill.

She seems to inform Polybotes about their force's current state.She also tends to call hot-dogs like Puppy-in-a-Blanket and when she offers one to Polybotes he first thinks she means an actual puppy, but later when she explains that is about an actual hot dog, he ignores it. During an assault on Camp Jupiter she and her sister are attacking the giant Eagles. They are both killed at the end of the battle, possibly by Mrs. O'Leary.

Personality
Stheno has a relatively carefree personality with a somewhat naive disposition. She is slightly friendlier than her sister Euryale, and is hurt when she told Percy that Medusa was the most hideous gorgon in their family. Ever since she "worked" at Bargain Mart, she has had a sales person attitude, trying to sell food at random times.