Theresia van Astrea (テレシア・ヴァン・アストレア) was Wilhelm van Astrea's wife and the previous generation's Sword Saint, who fought and single handedly brought an end to the Demi-Human War, roughly 40 years before the start of the main story. Though a minor character overall, she is pivotal to the character arcs of her relatives and later serves as a secondary antagonist in Arc 5.
After dying at the hands of the Hakugei 14 years ago, Wilhelm vowed to avenge her, which he does in Arc 3. However, in Arc 5, the Witch Cult reanimated her corpse and forced her to fight her own family. In the end, Reinhard put her out of her misery, an act that completely shattered his relationship with Wilhelm.
She was tall and had long red hair with flowers attached to it, blue eyes and a white complexion. She used to wear a white dress when operating as a knight of the Royal Guard, while in her free time she wore a green and red dress.
Theresia was described as an open-minded, sociable person who had a fatal flaw of shoving aside her own personal feelings while only focusing on those of others. Despite her immense talent and lineage, she never wanted to be a swordsman and take up the mantle of the "Sword Saint" due to a lack of interest and more over fear of her own power which easily stripped the lives from others regardless of her will. However, certain circumstances forced her to resign to her fate in order to end the civil war and protect the loved ones she had left. This eventually led Wilhelm to "take" the sword from her and free her from a life of bondage by duty. Theresia was an avid flower lover and could be found viewing them in ruins, where she and Wilhelm routinely met in her spare time.
Theresia was born to her father Veltol Astrea, and her mother, Tishua Astrea. Since she was born with a Divine Protection, she refused to use a sword, fearing hurting others. However, at the age of twelve, she obtained the powers of the Sword Saint from her uncle Fribal van Astrea. During the Demi-Human War, her uncle and her brothers, died participating in battles. Later, Theresia found a ruin, where she saw Wilhelm train. In each encounter, she asked him his reason for using the sword, to which he answered that it was all he had, but later he changed it, saying that he used it to protect others. Eventually, Theresia was informed of Wilhelm's achievements in the war and congratulated him.
One day, Wilhelm mentioned that the Demi-Humans were going to attack his home. She accompanied him to his village, and at the last moment saved him from being killed, defeating everyone before the reinforcements arrived. When he recovered, Wilhelm confronted her for hiding the truth of her powers. As he confronted her, she easily defeated him, before she states that she would not return. Since she had to protect him, she decided to participate and end the Demi-Human War. In gratitude a ceremony was made, nevertheless, a hooded man appeared, who defeats her in a duel. The person is revealed to be Wilhelm, recovering his responsibility to use the sword for being weaker than her, and they share their first kiss.
The happy couple got married and eventually formed a family, having a son, Heinkel Astrea, and eventually a grandson, Reinhard van Astrea. Later, everyone was informed that the Hakugei was approaching. Since Wilhelm was tasked with another mission, Theresia took the place of a fearful Heinkel to lead a group of knights to defeat the Demon Beast. However, during the battle, she lost her powers as the Sword Saint, encountered Pandora and passed away.
Divine Protection (加護 Kago): Theresia possessed two Divine Protections, the Divine Protection of the Death God (死神の加護 Shinigami no Kago) she was born with and the 'Divine Protection of the Sword Saint (剣聖の加護 Kensei no Kago) which she gained when she was twelve. With the former, slashes imbued with the Divine Protection's power leave curse wounds that are unable to be healed. It also increases the closer she is to the target, and she can even reopen wounds others have if they are close enough and were originally inflicted by her. If the wound expands so does the effect of the protection, if the wound was a light cut on the finger and the target cut off said finger this new wound becomes the target of the protection. The effects disappear if she dies.
- Sword Saint (剣聖 Kensei): The power of the Sword Saint gifted her with inhuman intuition and insight while maximizing her latent swordsmanship talent and knowledge of combat towards a level few could reach. This resulted in allowing Theresia to break down any kind of technique or attack directed at her while simultaneously showing gaps in them for the most optimal counter. These manifest as spectral "white lines" that only she could see which are implied to be guidance from the "Sword God". However, if someone possess equal or greater skill relative to hers the lines will distort making her opponents attack paths difficult to read.
Master Swordsman: Theresia was highly skilled at using the sword. Contrast to Wilhelm's deftier and measured style, her swordplay is more momentum based, employing a lot acrobatic flushes. It's described as if she is doing a beautiful deadly dance.
Equipment: As the Sword Saint, Theresia wields the Dragon Sword Reid (龍剣レイド Ryuuken Reido). The sword can only be drawn against opponents it deems worthy for it to be used against, being later given to her grandson Reinhard. After her death and being revived as a puppet of the Witch Cult, Theresia used a different sword in combat.
- Theresia was born on the June 12, Lovers day.
- When she was younger Theresia could not control the "Divine Protection of the Death God" and wouldn't be able to until much later in her life where she came to some terms with herself and that power.
- The author had confirmed that Theresia with her Divine Protection of the Sword Saint active would still be unable to defeat the Hakugei due to her having to look after her squad and protect them from the Whale's fog.
- Theresia is said to be in the upper middle part in terms of strength, out of all the Sword Saints that have ever lived.
- When she was young, she never bothered with studying magic, however, the author has stated that she most likely had an affinity with Yang Magic.
- Theresia's first words when she first met Reinhard were: "You are Reinhard, a child that's been blessed by your family. Please remember that before you learn about the world."
- Theresia van Astrea killed roughly ten thousand people, during the Demi-Human War.
- Theresia has her own named chapter in Arc 5, in both the Web and Light Novels.
- "Theresia had taken up her sword as the Sword Saint in the Demi-Human War and reaped countless lives. Hadn’t the number of people she’d killed neared ten thousand?" Sword Demon Battle Ballad Act 7: Part 1