Greta is a Ghoul found within the Underworld. She works alongside Carol in Carol's Place. She is apparently Carol's partner as she is jealous of her relationship with Gob whom Carol thinks of as a son. If the player is female Greta tells you to stay away from Carol.
Ahzrukhal wants the player to kill Greta in exchange for Charon's contract. If the player agrees with him, the best time to kill her is between 9-11pm, as this is when she steps outside of the Underworld for a smoke. Alternatively, the player can purchase the contract from him.