Wilhelm van Astrea, originally born Wilhelm Trias, emerged from the declining noble House Trias as its third son. He lived with profound emptiness until discovering his family's cherished sword, dedicating six rigorous years to its mastery. This relentless training forged a legendary swordsman whose ferocity during the Demi-Human Alliance War earned him the epithet "Sword Demon." His path irrevocably shifted upon encountering Theresia van Astrea, the reigning Sword Saint, near a field of daisies. Though resentment initially filled him after she saved him during an attack on his homeland, Wilhelm vowed to free her from the burdens of her title. After two intense years of training, he defeated her in a duel at the Royal Palace of Lugunica, compelling her retirement. He married her shortly thereafter despite opposition from her father, Veltol Astrea. Their marriage was marked by deep mutual devotion, with Theresia often associated with yellow daisies reflecting her love for gardening. Wilhelm further demonstrated his combat prowess during the Silver Flower of Picoutatte, defeating the Eight-Armed Kurgan, a champion serving the Sin Archbishop of Pride, Stride Vollachia. Later, during the Subjugation of the Evil Dragon, Stride kidnapped a pregnant Theresia. The climactic battle saw Stride perish. Theresia gave birth to their son, Heinkel Astrea, and Wilhelm was appointed Captain of the Royal Guard for his heroism. Tragedy struck fourteen years later when the White Whale, one of the Three Great Mabeasts, resurfaced. Heinkel refused to confront it, prompting Theresia—who had lost her Sword Saint blessing to their grandson Reinhard—to take his place. She perished alongside the expedition force in the event known as the Great Expedition. Consumed by grief, Wilhelm blamed Heinkel for cowardice and Reinhard for "stealing" Theresia's Divine Protection, though Reinhard was a child at the time. He abandoned the Astrea family, dedicating himself solely to avenging Theresia by joining Crusch Karsten's faction in the Royal Selection. With strategic assistance from Natsuki Subaru, Wilhelm finally slew the White Whale, gaining closure for his wife's death. This victory came at the cost of Crusch's memories being consumed by the Sin Archbishop of Gluttony. A year later, Wilhelm attempted reconciliation with Reinhard, acknowledging three decades of neglect. This effort shattered during the Watergate City incident when the Witch Cult reanimated Theresia's corpse as a weapon. Forced to confront his reanimated wife, Wilhelm was ultimately saved by Reinhard, who delivered the fatal blow to Theresia. Her second death extinguished any hope of familial reconciliation, leaving Wilhelm and Reinhard irreparably estranged. Wilhelm's combat abilities evolved significantly across his life. As a youth, he utilized the Flow Method to amplify physical capabilities via mana, achieving superhuman strength, speed, and durability. His skills included analytical prediction, instinctive reaction, and extrasensory perception, allowing him to sense hostility and evade invisible attacks. He could slow his perception of time during combat and interact with intangible entities, such as cutting through the Unseen Hand. In his prime, these abilities intensified: he accelerated his development through battle, mimicked opponents' techniques, and displayed surgical precision—exemplified by bracing a dragon's jaws with crossed blades. His aura also grew overwhelming, capable of unsettling even seasoned warriors. As an older man, despite physical decline, his strategic intellect and swordsmanship mastery remained formidable, refined through decades of experience.