She is one of the sea monsters guarding the Bermuda Triangle, also known as the Sea of Monsters, along with Scylla. All the boats that have gone into this sea ended up swallowed by her.
After Thalia's tree has been poisoned by Luke Castellan, Clarisse La Rue is sent to Polyphemus's island in the Sea of Monsters to get the Golden Fleece, which has the power to heal anything. When she arrives, her ship is attacked by Scylla and is later swallowed up by Charybdis.
Some times later, Percy Jackson, his brother Tyson and Annabeth Chase come into the Sea of Monsters. Shortly after their arrival, Charybdis's mouth emerges from water and swallows their boat. In the stomach, they meet Clarisse and her crew, who are about to be grinded. However, Percy gets the idea to use their ship's gun to shoot in the stomach, creating a hole large enough for the ship to pass through. Clarisse then manages to lead her ship out of Charybdis's belly and after resurfacing they move away from the sea monster.