Illyasviel is an homunculus created by the Einzbern family to serve as the vessel of the Holy Grail in the fifth Holy Grail War and is also the Master of Berserker. She was given the task to win the Holy Grail War and obtain the completed Holy Grail for the Einzbern family. Illyasviel is cheerful, energetic, and friendly. She is very playful with people who she considers friends but is ruthless in battle.
Illyasviel lives in an enormous mansion in a forest at the outskirts of Fuyuki City with her 2 maids, Leysritt and Sella, and her Servant Berserker. She occasionally steals away from her home to roam the streets of the town and enjoy the winter. Illyasviel is actually older than Shirou, but her physical appearance remained that of a child because of her being an homunculus.
Illyasviel also co-hosts the Tiger Dojo with Taiga Fujimura, the hint corner of the Fate/Stay Night visual novel, where she is nicknamed "bloomer".
Around 200 years prior, the Tohsaka family, Makiri family, and Einzbern family worked together to initiate the Fuyuki Holy Grail War. The Einzbern family specializes in the Third Sorcery, the ability to give physical form to a soul. They were responsible for providing the vessel of the Holy Grail. As the years past, the Einzbern family wished to rediscover the technique behind the Third Sorcery, which is their main motivation for winning the Holy Grail War.
Some time before the fourth Holy Grail War, Illyasviel was created to be the vessel of the Holy Grail in the fifth Holy Grail War. She was acquainted with Kiritsugu Emiya who was hired by the Einzbern family to win the Holy Grail War on their behalf. Kiritsugu later discovered that the Holy Grail was tainted by the Servant Avenger, Angra Mainyu during the third Holy Grail War and ordered his Servant Saber to destroy the Holy Grail to prevent Angra Mainyu from gaining form and being unleashed.
Illyasviel and the Einzbern family viewed this as an act of betrayal and Illyasviel grew to hate him. She later leaned about Kiritsugu adopting Shirou and she developed a love-hate relationship with him, both as an extended family and as the successor of her enemy.
Some time before the fifth Holy Grail War, the Einzbern family summoned the Servant Berserker for Illyasviel. She would serve as both the vessel of the Holy Grail and the Einzbern family's representative in the Holy Grail War. Berserker's immense power and energy consumption posed a tremendous strain on Illyasviel and she suffered greatly.
Illyasviel was sent to Fuyuki City along with 2 maids in order to participate in the Holy Grail War.
Illyasviel and Berserker ambushed Shirou and Rin shortly after their briefing with Kotomine. She sent Berserker to attack Saber, easily overpowering her. Berserker was about to land his final blow on Saber but Shirou took the attack in her place. Illyasviel was stunned by his foolishness and left soon after.
After Saber's fight against Rider, Shinji ran into Illyasviel and she ordered Berserker to kill him. Later, she met Shirou in town and kidnapped him. Saber, Rin, and Archer came to rescue him. Illyasviel sent Berserker to attack them and Archer chose to fight against Berserker to allow the others to flee. Archer managed to take off 6 of Berserker's lives before being defeated which shocked Illyasviel.
The next day, Shirou, Saber, and Rin fought against Berserker. Rin took off 1 of Berserker's lives and Illyasviel explained that he has 12. Shirou then projected a copy of Saber's first sword, Caliburn to take off another 7 lives, defeating Berserker. Illyasviel was stunned by her loss and fainted. Shirou then carried her to his home and insisted that she stays.
Illyasviel grew attached to Shirou, much to Saber's and Rin's dismay. She managed to warm up to Saber. Near the end of the war, Gilgamesh attacked Shirou's home, kidnapping Illyasviel to unleash the evil within the Holy Grail. Shirou managed to defeat Kotomine and rescued Illyasviel.
Unlimited Blade Works
Illyasviel ambushed Shirou like in Fate, but Saber brought the fight to a nearby graveyard where she had terrain advantage over Berserker because her smaller size allowed her to navigate around the tombstones, while Berserker's movement was greatly diminished. Archer then fired a Caladbolg II at Berserker, severely damaging him. Illyasviel acknowledged Archer's power and retreated.
Much later after Caster stole Saber and Archer betrayed Rin to join Caster, Shirou and Rin decided to ally with Illyasviel to fight back. At the Einzbern castle, they discovered Shinji and Gilgamesh. Gilgamesh defeated Berserker with several showers of swords and Enkidu then ripped Illyaviel's heart out and left her for dead.
Illyasviel ambushed Shirou like in Fate as well. Later, Shirou found her in town and they became friends. She explained that she will only fight him at night. Illyasviel explained that Kiritsugu was her enemy and later showed Shirou the way to her castle via far-sight.
After discovering that Sakura was held hostage by Zouken's worm familiars and was forced to fight, Shirou talked to Archer and Illyasviel. Illyasviel explained that she didn't like Kiritsugu because he abandoned his humanity for the sake of his ideals, the same ideal that Shirou had adopted. Shirou then decided to discard his ideals to save Sakura.
Later, Shirou decided to visit Einzbern castle to warn Illyasviel about Zouken and the black shadow. There, he witnessed Saber Alter defeating Berserker and he was devoured by the black shadow. Shirou covered Illyasviel from the black shadow's explosion which took his left arm. When Archer offered his arm to save Shirou, Illyasviel mentioned that she knew about the relation between Shirou and Archer. Illyasviel and Rin then moved in with Shirou to protect each other, both having lost their Servants.
After Zouken revealed that he turned Sakura into a Holy Grail, Illyasviel explained that Sakura had been acting as the Holy Grail in the war, that she had been absorbing the magical energy of the devoured Servants and will eventually break down from the stress of containing all that power.
After Sakura embraced Angra Mainyu, she attacked Shirou's home in order to retrieve Archer's spirit from Illyasviel. She convinced Sakura to return to Einzbern castle explaining that she needed her ceremonial dress to give form to Angra Mainyu. The following day, Shirou and Kotomine rescued Illyasviel. She witnessed Shirou using Archer's arm for the first time to defeat the corrupted Berserker.
Illyasviel then explained the details behind Angra Mainyu, the Third Sorcery, and the Holy Grail. The Servant system uses an incomplete form of the Third Sorcery, since the Servants are not able persist in this world independantly. The Great Holy Grail is an enormous reserve of magical energy. It provides the energy to summon Heroic Spirits and the Holy Grail is the key to gain access to the point of the origin of everything, which would give a magi untold power. The promise of granting a wish was made to entice magi and Heroic Spirits to participate in the war.
Angra Mainyu was an ordinary man who was made a scapegoat by his village, piling on all their sins on him and treating him as a being of pure evil. The Einzberns summoned him during the third Holy Grail War thinking that he had the power to win the war. He turned out to be incredibly weak and was defeated easily. When he was fed to the Holy Grail, it gave him power and turned him into what the villagers wished for, an all powerful being of pure evil. That in itself was the Third Sorcery in its complete form. Angra Mainyu then tainted the Holy Grail from then on.
Ri requested Shirou to project a copy of the Gemmed Sword of Zelretch for her to wield against Sakura. Illyasviel gave Shirou access to the memory of her ancestor, Justeaze von Einzbern during the founding of the Fuyuki Holy Grail War where she was with Kischur Zelretch Schweinorg when he still had the sword. Shirou analyzed the sword and projected a copy of it.
In the true ending, Illyasviel wore the Heaven's Feel dress and went to the site of the Great Holy Grail. Illyasviel stopped Shirou from projecting past his limit and told him that she was both his older and younger sister. She then performed an incomplete Third Sorcery on him to preserve his soul and sacrificed herself to seal off the Great Holy Grail for good.