Melinoe

Melinoe is the goddess of ghosts, sometimes confused with Hecate. Her sister is Macaria.

The Demigod Files
Melinoe shows Ethan Nakamura the way out of the Underworld, and takes on the appearance of Thalia and Nico's mothers to confuse and torment them, but they do not fool Percy, who breaks the illusion. He swings at her with his sword, and she dodges, only to be hit by Thalia with an arrow. She then vanishes into the fog.

Appearance
Melinoe is a strange goddess, and her appearance is frightening. Her left side is black and hardened like a mummy, and her right side is pale and chalky as if she were drained of all her blood. She wears a golden dress and a golden shawl, and her eyes are empty black voids. However seeing her in this form is unlikely, being the goddess of ghosts to demigods (and presumably anyone with ghosts) she appears as the most tormenting version of them, haunting people in there nightmares and in person if ever directly confronted. The illusion is so strong that even when normal, her immediate victims still see her as the ghost.

Powers
Melinoe can take on the appearance of the dead people in a person's life, using guilt and anger from the person as an ally. She then uses this to destroy them.

Personality
Melinoe is dissatisfied with Hades' rule over the Underworld, and the fact she cannot haunt the mortals of the human world every day. She opts for Kronos, as she is promised that she will be able to haunt whenever she wants.