Mary Sue

Mary Sioux or Mary Sue is one of the main antagonists of the series, the ultimate rival and the tool used by Being X to counterbalance Tanya Degurechaff in the war. She is the sole daughter of Anson Sioux and his wife, who hail from the Unified States.

Appearance
In the anime, Mary is a young girl with long brown hair with curled ends and green eyes. When she was still an ordinary citizen, she was seen wearing a blouse with a long skirt.

In the light novel, she has straight and long brown hair, and her eyes were greenish blue. As a volunteered soldier, she is seen wearing a US military uniform.

Personality
When she was a citizen, she was a caring and loving girl. She loved her father very much.

After her father's death, 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.

Past
Mary lived in a quiet, simple and average life with her mother. Sometimes, she had slight struggles without her father, which left her side to become a soldier to benefit her country.

Battle of the Fjord/Plus Ultra; Chapter IV: The devil in the coast of Norden
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 stolen and kept by Tanya during the battle of Fjord.

How to use a Victory/The Finest Hour; Chapter V: Internal affairs
After Mary learned about his father's fate in her grandmother's house in Arkansas, she volunteered to become a soldier to take revenge on the Devil of the Rhine even though she was below the minimum age for volunteer soldiers. At that moment, she prayed to the God and received the same blessing as Tanya from Being X, albeit with some key differences.

Abilities
Prayer/Blessing/Gospel Magic

The user can cast a spell by praying to Being X stronger than Tanya's powers. The speed is faster than an average human. In addition, the firing abilities exceed Tanya's capabilities with 100% exact hit to the target.

She can blow a whole battalion (48 men) with only one artillery spell, as opposed to Tanya's, which can only wipe out a whole company (12 men). However, it reduces her mana to fifty percent of her full power, as shown during her first mission as a volunteered soldier of US.

Rage Mode

The user's hatred and a grudge against "Devil of the Rhine" become the source of the power. It exceeds the limits of Blessed User. The operation orb of the user becomes brighter and regenerates mana quickly.

However, as a side effect, the user loses control of their own movements and reason.

Quotes
Catchphrase: "Curse you, Devil of the Rhine!"

(Mary's declaration [Episode 12]) : "I declare that for the sake of the peace I must protect and for the sake of the family I love, I will give my whole power! To make a world that no one else has to experience the sadness of losing their family to the Empire. And to enact God's justice! I swear by my good heart which believes in the Lord! May the Lord's grace be with me!"

(Mary to Tanya) : "I finally found you! Devil of the Rhine!"

Trivia

 * Her name "Mary Sioux" or "Mary Sue" comes from the trope Mary Sue. However, because her character was rewritten when Youjo Senki became a light novel, she doesn't fit the trope very well anymore.
 * She is the only person who can match Tanya von Degurechaff in many battles.