Mary Sioux is one of the main antagonists of the series, the ultimate rival and the tool used by Being X in order to stabilize a neutral war against Tanya Degurechaff. She is the sole daughter of Anson Sioux. After her father's death, she became rather more vengeful and serious. Her hatred grows when she hears the name "Devil of the Rhine," she will begin to rage but resulting exposing herself in a controlless side. She is the only person who can match Tanya von Degurechaff in many battles.