Izumi

The gold King kept the Slate under his control for over half a century. But he did not do it alone, instead he had help.

Momozawa Izumi is the whole reason why the gold King has been able to keep the Slate under his control. Made from the power of the Slate, Izumi can control and create auras of all the Kings. Supposedly, she's stronger than all of them.

But after the Kagutsu Carter incident, Izumi disappeared.