Martha

Martha is a snake that was caught by Hermes. She is immortal, though she is not a Monster. Demigods and Gods can hear her in their heads. She is always seen with George.

Description
Martha is constantly scolding George for whatever he said. They live on Hermes' Caduceus. When it is a phone, she is on the antenna.

The Sea of Monsters
In The Sea of Monsters, Percy goes to the lake. Hermes comes, and Martha is with him. She is constantly scolding George. At the end, she scolds George for teasing Percy.

The Last Olympian
In The Last Olympian, Martha comes with Hermes to Olympus to deliver a message. George and Martha tell him not to kill Percy.