Mary Sue

Mary Sue or Mary Sioux is one of the main antagonists of the series. She is the sole daughter of Anson Sue and his wife, who hailed from the Unified States.

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

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 manga
Mary has long and wavy hair with a hairband. She wears a dress, which has much lace on.

Personality
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 mom, 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.

Past
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.

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 captured 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. 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
(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!"

The light novel
She got the submachine gun from an unnamed man came from UK. She has high aptitude for magic inborn like her father, and no clear evidence proofs that she received more power from gods. She joined the volunteer army more for the patriotism than the hatred since she didn't know who killed her father. She finally knew it by identifying the signature on her father's gun taken by Tanya.

The anime
She is fashioned as the ultimate rival and the tool used by Being X to counterbalance Tanya Degurechaff in the war. The anime didn't point out how she got the submachine gun. She also seemed to receive the power from Being X when making the declaration of volunteer.

The manga
She got the submachine gun from Uncle John (his pseudonym). When she went to Unified States with her mother, she received three miracles from gods.
 * As her father's prayer, her future has been blessed. Like an étoile in ballet, or a prima donna in opera, she will have an unbreakable will and a glittering destiny.
 * As her mother's prayer, she is protected by the miracles. Swords will break before her, bullets will dodge her, fire won't reach her, and her wounds will heal. Nothing short of a miracle of the same scale can break it.
 * As her prayer, she gains strong magical power and sufficient mana like her father, so that she can fight for the motherland with her father.

Trivia

 * Her name "Mary Sue" or "Mary Sioux" 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.