Caster’s True Name is Illyasviel Emiya, known in life by many as the Second Magus Killer, the Queen of Steel, and the Silver Huntress. As with other members of the Emiya-Einzbern family inscribed in the Throne of Heroes (like EMIYA [Archer], EMIYA [Assassin], and Justeaze [Ruler]), her story is deeply entwined with the Holy Grail War.
Born in Germany in 1986 to Kiritsugu Emiya and Irisviel von Einzbern, she was to be the incarnation, the human sacrifice which contained the Lesser Grail for the 5th Holy Grail War should it ever have begun. While raised by Kiritsugu and Irisviel during her first 8 years of living, she was abruptly sent back to Germany in the midst of the chaos of the 4th Holy Grail War, in which Irisviel was sacrificed to become the Lesser Grail, while Kiritsugu Emiya fought a bitter battle with Kirei Kotomine, ending in a draw which coincided with the destruction of much of Shinto, Fuyuki.
The patriarch of the Einzbern family, Jubastacheit von Einzbern, sought to ‘train’ Illyasviel, by getting her to believe that Kiritsugu had abandoned her and killed her mother, then using her to get revenge on Kiritsugu Emiya for destroying the Grail and betraying the Einzberns. However, in the aftermath of the Fuyuki Fire, Kiritsugu isolated himself from the world, and began to prepare a mission to retrieve Illyasviel and save her from the Einzberns. In the year between the fire and her retrieval, Illyasviel’s conviction and belief that Kiritsugu would save her let her resist the callings of Angra Mainyu, appearing to her as a mirage in the form of Irisviel.
While Kiritsugu saved Illyasviel, it came at a great cost. Kiritsugu’s body was rapidly drained, stretching the sheath of King Arthur, Avalon, and its healing to its meagre limits. Angra Mainyu decided to spare Kiritsugu, but burnt down the Einzbern mansion in Germany in a manner similar to the Fuyuki Fire. And Illyasviel, in the middle of all of this, had her perception of the world as a whole shifted and distorted, as the fires blazed around her.
In the interim time between the 4th and 5th Holy Grail Wars, from 1995-2004, Illyasviel lived in Fuyuki. Kiritsugu was able to teach Illyasviel all he could before he died in 1999, and she adapted accordingly into the Fuyuki environment, shedding the Einzbern name for the Emiya one. She met Rin Tohsaka, a fellow magus, who, unaware of Illyasviel’s true heritage, became well acquainted with her.
During the interim, Illyasviel also received the Emiya family crest from the Clocktower, who entrusted the crest in exchange for Illyasviel attending classes over the summer. She met Waver Velvet, or Lord El-Melloi II, who told her of the events of the 4th Holy Grail War, and Luviagelita Edelfelt, a young Finnish magus that reminded Illyasviel of Rin.
When the 5th Holy Grail War starts its countdown to begin, Illyasviel received her Command Seals first. She prepared accordingly in the month or so that she had, and trying to come up with plans. Unfortunately, all of them were negated almost instantly, when on the night before Illyasviel planned to summon her Servant, Servant Berserker broke through the Tohsaka residence’s Bounded Field and attacked Rin. This caused Illyasviel to summon her Servant early, summoning Servant Saber without properly preparing her resources. This caused Saber to be greatly weakened, though her power still outweighed the incarnation of King Arthur summoned by Shirou Emiya.
Illyasviel’s plans were further derailed, when she met Miyu Aoki, an accidental Master brought into the War due to Berserker killing her, then Miyu being revived by Rin’s last act of magecraft (the pendant), and then summoning Servant Caster when cornered by Berserker. Illyasviel began to have romantic feelings towards Miyu, and ordered Saber to refrain from attacking her. The two fought in the War with their Servants, both managing to kill Berserker in a pivotal battle in the Fuyuki Forest.
However, with the intervention of Sakura Kotomine into the War directly, the entire dynamic of the War changed. Sakura fought enemy Servants alongside Berserker, killing Luvia and Assassin in a rout that leveled much of Homurahara Academy. She fought Rin and Archer at Ryuudou Lake, killing Archer with Berserker, and then killing Rin with her own hands. Eventually, it reached a point where the remaining Masters and Servants were unable to directly fight Sakura and Berserker, who decided to go into hiding and bide their time. The War reached a 3-week long stalemate, which was broken when Sakura attacked the abandoned Einzbern estate that Miyu and Caster had set up base. However, when Illyasviel realized that Miyu was being attacked, she rushed to go help.
In the ensuing battle, the entire mansion was leveled. Saber managed to defeat and kill Berserker, but Saber was killed trying to defend Illyasviel from one of Sakura’s desperate attacks. Meanwhile, Sakura and Caster reached a stalemate, one that Sakura won, thanks to Caster’s mana being expended. Illyasviel, in utter rage, had an epiphany in the moment, and embraced a vision that had once been just a dream and memory: a winter landscape bathed in fire and covered with steel. Unleashing all of her mana reserves, she attacked Sakura, and killed her.
Illyasviel proceeded to do the same to the other Masters of the War, trying to prevent further attacks on Miyu, who she entered a true romantic relationship with during the stalemate. However, all of Illyasviel’s actions against the other Masters became violent, heartless, and to the point, repulsing her lover so much that Miyu begged her to stop fighting. When Illyasviel refused, Miyu realized that Illyasviel had devoted herself to the idea of love to the point of murder. At that moment, Miyu broke up with her, and later left Fuyuki. The mere action caused Illyasviel’s heart to shatter, and pushed her to embrace her mind of steel.
She ended the 5th Holy Grail War, by killing or removing every other Master left after Sakura’s massacre. After the War, she left Fuyuki, and embraced the only dream that she has left: to be a superhero.
Over the 34-year span between the end of the 5th Holy Grail War and her death, Illyasviel became the reincarnation of Kiritsugu Emiya. She killed, murdered, and slaughtered thousands, just to save handfuls of people. Unlike Kiritsugu Emiya, who had a dream of an endless utopia, Illyasviel Emiya had no ambition, no dream to speak of, for she had already achieved it. She simply picked up her weapons, fought, then returned to a life of endless death.
During her career as an assassin, she gained possession of Kanshou and Bakuya, which, in another world, had ended up as the embodiment of another Emiya’s life. However, during her possession of them, she honed them to be completely separate, serving different purposes and serving different ways how to kill. She (unknowingly) used the concept of her Reality Marble to influence and alter the twin blades, eventually reaching a point where the blades that had been married and would return to the wielder’s hand should the other be held in the other would utterly destroy the wielder or target would they be in contact with both blades.
In both the normal and moonlit worlds, she became famous as a symbol of justice, a shining light that would kill those who sought evil. Due to the sheer mystery surrounding her operations, and the suspicious lack of memory that witnesses to Illyasviel’s missions had, she became an enigma, a figure that transcended normal humanity.
As the years wore on, they passed as she killed and killed, all in the name and pursuit of being a hero. However, just as it had with her father and the Shirou Emiya of another time, she was destined to break. For her, it was a mission to Fuyuki thirty years after the 5th Holy Grail War that would carve Illyasviel Emiya’s name into the names of legend, a legend of a tragic hero, a mission that would haunt her to the grave.
During a mission to take out a rogue group of mages who had holed up in the slums of Old Fuyuki, the mages took a group of hostages to try and dissuade the Magus Killer from killing them. However, in spite of this, Illyasviel ignored this, and stormed their headquarters underground. When she reached the leader of the mages, he proceeded to threaten her with a final hostage, whose head was bagged, one who looked vaguely familiar to her. Despite this, she shot the mages’ leader in the face, but not before the hostage’s throat was slashed. As she went to untie the hostage’s head from the embrace, she took a look at the hostage’s eyes. Then, he began to scream, realizing that she let her former lover, Miyu Aoki, die by foreign hands.
After this incident, she became reclusive, fully dedicating herself to reaching the Fifth Magic by way of the Emiya family Crest. However, when word of her research began to spread, the Mages’ Association became worried, and when Illyasviel was shown to have mastered the ability to alter time outside of her own body, placed a Sealing Designation on Illyasviel.
Finally, 4 years after that fateful day, the Association striked. 700 Enforcers and magi were sent, to quell the Magus Killer who ‘seeked’ Akasha. Sieging the Einzbern mansion outside of Fuyuki, they surrounded Illyasviel in the place she once called home. For 10 days, Illyasviel held out, betrayed by colleagues and acquaintances alike, as magus after magus fell to the complex labyrinth of traps and defenses that she had planted as a security measure. When the Enforcers finally broke through, 400 of their own had already fallen to the defenses of the Einzbern mansion. When they reached Illyasviel herself, she fought the rest of them as they charged through the front. Even as her body began to fall apart, with Mystic Code after Mystic Code killing magi and then disintegrating, with her determination hell-bent on killing her enemies, she still took them head on.
And she succeeded. Despite being impaled multiple times in the heart after having used all of her mana and most of the leeched mana from the Fuyuki leylines, Illyasviel killed the last Enforcer standing with a shoddily crafted Projection of Kanshou Alter, as she died.