Myrrh is also known as the the Woman of Mystery or the Scary Lady, her true identity and history are only partially known. She shares her bed with Frederick. She is a woman of considerable talent and knowledge, both magical and otherwise. She apparently played some role in raising Sylvester.
The Scary Lady is a demon, accidentally summoned from another world by Frederick, a skilled wizard and member of the E Family. Frederick managed to magically bind her (a binding physically represented by an unremovable necklace), losing his powers in the process. In a possible case of Stockholm Syndrome, she became romantically involved with Frederick.