Theresia van Astrea (テレシア・ヴァン・アストレア) was the former Sword Saint and Wilhelm van Astrea's wife and the hero of the civil war 40 years prior to the main story. She went missing while fighting the White Whale 14 years ago, but was later seen as a resurrected corpse fighting for the Witch Cult. At the end of Arc 5, she was successfully killed by her grandson, Reinhard van Astrea.
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 white 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 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 engaged in another mission, Theresia took the place of a fearful Heinkel to lead a group of knights to defeat the Demon Beast, whose attack was orchestrated by Pandora. However, during the battle, she lost her powers as Saint of the Sword, and passed away.
Many years later, Theresia was somehow resurrected by the Witch Cult. Eventually, when the Witch Cult arrived in Priestella, the Sin Archbishops took control of the city, Thearesia was also part of this, however, Subaru organized a plan with his friends to defeat them. Suddenly, she fought against Wilhelm, only to be quickly defeated, but she soon counterattacked, then noticed Heinkel as her next victim, but Reinhard interposed. Because he considered her a defiled corpse, he defeated her mercilessly with a single blow. Already on the ground, Theresia recovered her reason in the arms of Wilhelm, a moment in which they confessed their love, and while she turned to ashes, she mentioned how happy she would be if he reconciled with Reinhard.
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. She also wielded the Holy Sword Astrea (聖剣アストレア Seiken Asutorea), her favorite sword, which was later inherited by her son, Heinkel Astrea.
- 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 all her Divine Protections active would still be unable to defeat the White Whale 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.