Secret

Greta Hayes was Harm's sister.

Early life
Greta was Harm's younger sister. She was the only thing he loved, but since he wanted to be pure, he had to cut her out of his heart—by killing her. Artemis speculated that the last thing Greta saw was the broken neon sign of Abel's House of Secrets across the street of her the backyard.

Somehow, she became a ghost.

Present
On Halloween, Greta helped Artemis and Zatanna escape from Harm, leading them to his home, where Harm became suspicious and, rather than kill them as planned, took them captive.

Greta reappeared in the room in which Zatanna was held. She removed her gag, allowing Zatanna to free herself with her magic. Zatanna questioned Greta's motivation, but only got "Secret" as a response.

After Zatanna and Artemis escaped from Harm, she came out of her grave in the backyard again. She showed them how her brother had killed her. She was the only thing he loved, and thus a weakness in his thirst for power. When Harm found them, she charged at him and took his heart away. This made him lose control of the Sword of Beowulf, and Artemis was able to defeat him.

With Harm down, Zatanna promised Greta a proper burial.

Powers and abilities
Greta had no apparent powers before dying. After her death, she became a ghost. She was intangible, could float, and disappear at will. She was also able to make an image of the dagger used to kill her and was, at least, able to touch small solids, such as Zatanna's gag and push a small fire escape.