Mary Sue

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

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 long skirt.

In the light novel, she has straight and longer brown hair, her eyes were greenish blue. As a volunteered soldier, she is seen wearing an 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 when she hears the name "Devil of the Rhine", she will begin to rage but resulting exposing herself in a controlless side.

Past
Mary lived quite a 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 modernized Tommy gun for Anson as an early Christmas present. 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
Mary volunteered to become a soldier to take revenge on the Devil of the Rhine regardless she wasn't at a minimum age for volunteer soldiers. At that moment, she prayed to the God and received the same blessing as Tanya from Being X.

Abilities
Prayer/Blessing 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. Also, the firing abilities exceeds to 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 an volunteered soldier of US.

Rage Mode

The user's hatred and grudge to "Devil of the Rhine" becomes 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" comes from the biblical character named "Mary of Nazareth".
 * She is the only person who can match Tanya von Degurechaff in many battles.