Mary Sue

Mary Sioux a.k.a Mary is one of the main antagonist of the series, the ultimate rival and the tool used by Being X inorder to stabilize a neutral war against Tanya Degurechaff. She is the sole daughter of Anson Sioux.

Her real name is Mary Sue, which makes "Sioux" mistranslation.

Trivia
Her name "Mary" comes from the biblical character named "Mary of Nazareth".