Mary Sioux is a major recurring character of the light novel/anime series Youjo Senki: Saga of Tanya the Evil. She is the commander of the second lieutenant of the Legadonia Federation. She is also the daughter of the late Anson Sioux. After her father was killed by Tanya von Degurechaff, Mary volunteered to become a soldier to exact her revenge on her. She is presumably going to be the main antagonist of the upcoming film based on the anime.
She is voiced by Haruka Tomatsu in the Japanese versione of the anime, and by Tia Ballard in the English version of the anime.
Mary lived in a quiet, simple and average life with her mother. Sometimes, she had slight struggles with her father, which left her side to become a soldier to benefit their homeland.
Before the Christmas in 1924, Mary bought a state-of-the-art submachine gun for Anson as an early Christmas present. The novel and manga revealed that she received it from an agent by the name of John who was working for the Allied Kingdom. The gift soon was captured and kept by Tanya during the battle of Fjord.
After Mary learned about her father's fate in her grandmother's house in Arkansas, she volunteered to become a soldier. At that moment, she prayed to the God and received the same blessing as Tanya from Being X, albeit with some key differences.
When she was a citizen, she was a caring and loving girl. She loved her father very much. Though her father's death almost crushed her mother, she joined the US volunteer forces more for patriotism than taking revenge to the Imperial soldier who killed her father.
After she identified her submachine gun on Tanya's hands, she became rather more vengeful and serious. Her hatred grows whenever she hears the name "Devil of the Rhine", and she will begin to rage, resulting in her exposing herself in an out-of-control state.
When she was Drake's subordinate, he found her naive and stubborn, which usually screwed up their efforts and enraged him.