As a homunculus of the Einzbern, Irisviel possesses a genetic resemblance to her ancestor, Justeaze Lizrich von Einzbern, who formed the core of the Holy Grail when the Holy Grail Wars began. Her silver hair and crimson eyes are distinctive traits of Einzbern homunculi, such that Tohsaka Tokiomi recognizes right away that she is a member of the Einzbern family.
Because of her striking appearance, she tends to stand out, despite her attempts to dress informally. Irisviel dresses in long white gowns trimmed in gold in the Einzbern castle, but wears more functional clothing when she goes to Japan. Her clothing choice, though considerably less formal than her usual garb, is still considered by ordinary people to be high-end fashion. Irisviel wears a burgundy blouse with a brooch, a white skirt, thigh-high white boots, and navy coloured tights; in cold weather, she also wears an elegant fur-trimmed white coat extending to her knees and a white fur cap.
Because the plan of conceiving Illyasviel was decided after Irisviel was coined, her pregnancy of Illyasviel was not much of a natural development since it was an additional role imposed onto her that she originally was not supposed to have. An exception among Einzbern homunculi whose functions are determined when they are designed. Such an exception happened due to Jubstacheit von Einzbern’s judgment that doing so would be more efficient to nurse a child rather than producing a homunculus. Thus, despite being a homunculus, Irisviel had a human woman’s experience of giving birth to and raising a child (similar to herself) as a mother, and this has a greater influence over her feelings than Jubstacheit expected. Made to be a puppet with no desires, she managed to find happiness in her existence when she gave birth to Illyasviel. Furthermore, 9 years is a long time to maintain a relationship with a human. Before her there were no precedents of an Einzbern-made homunculus with such colorful and eccentric personality. To put it bluntly, she was the first Einzbern homunculus to be treated like a human being and learn human emotions to such an extent. Originally an existence that surpassed humans solely through the functions of mind and body, her personality did not collapse even after going through an uncommon development, and she managed to attain self-consciousness without any feelings of inferiority just like any human being. Irisviel is innocent and carefree, which results from living her entire life without leaving the Einzbern castle in Europe until the beginning of the Fourth Holy Grail War. Coupling the inborn knowledge and reasoning of homunculi - which are crystals of the 1000 years of the Einzbern - plus the fact that her life experience is that of a mere 9 years, in the end she became a princess that has both the elegance of a lady and the childishness of kid. Despite the way Irisviel acts, she displays a mature attitude and understanding towards Kiritsugu and the agony he suffers as well as the knowledge that she will likely die in the war because of his actions. She also tries to present Saber with a better impression of Kiritsugu since Saber feels her Master's cold nature derives from his belief she is unsatisfactory as a Servant because she is female. She approved of Kiritsugu’s ideal and vowed to sacrifice herself for its sake, but the truth is that she never actually understood that ideal. In fact, she just wished to walk the in the same path together with the person she loved, but – in order not to burden Kiritsugu – she decided to behave as a woman who would die for that same ideal rather than just a woman that would die for her husband. Although she wished for Kiritsugu and Saber to keep winning and end the fighting, she had little desire towards achieving the Third Magic, which was the Einzbern family’s goal. The Magecraft that she inherited involves the creation and forging of matter, as well as its applications. Her grade as a magus surpasses that of Kiritsugu. Her weapons are thin, long and flexible wires, which she uses to either produce an instantaneous homunculus with alchemy that attacks the enemy while changing itself into numerous shapes. She may also use her weapon to immobilize her target.
During the 4th Holy Grail War, she became the provisional Master of Saber while relying not on the Servant System, but simply through the vows of chivalry. Saber's actual Master is Kiritsugu, the Einzbern family's representative in that war who decided to act independently from his Servant and arrived in Fuyuki City without her. After Emiya Kiritsugu summoned Saber, he made Irisviel hide Avalon – which had served as a catalyst to the summoning – inside her body so to serve as insurance while she performed her act of fake Master. Irisviel’s and Saber’s role at the time was to openly take action and serve as a decoy in order to lure out enemies, while Kiritsugu worked in the shadows with his assistant, Hisau Maiya. Due to her secluded and isolated upbringing, she became fascinated with Fuyuki City and openly explored the city with Saber as her escort during her first day. Incidentally, the fact that Saber, who could not dematerialize her spiritual body, disguised herself as a man at that time was mainly due to Irisviel's tastes, proof of how her carefree nature extended itself to her relationship with Saber. The two got along well and became close enough friends for Saber to be willing to tell the story of her past to Irisviel. Like the Einzbern Masters of the past, Irisviel was skilled in Magecraft but was not suited for combat, making her generally left to be an observer whenever Saber entered in battle. At the same time, she also relayed information to Kiritsugu through technological means, such as tracking devices and cell phones, which remained undetected by other Masters. She and Saber met with Kiritsugu and Maiya a limited number of times during the war to decide which Masters and Servants to focus their attacks on, but rarely worked together. After Assassin, Caster and Lancer were defeated, she agreed with the ceasefire agreement proposed by Tohsaka Tokiomi under the terms of “offering information about Rider and Waver Velvet’s hideout” and “Kotomine Kirei’s deportation to overseas”. As the vessel of the Holy Grail, Irisviel grew increasingly weaker as the Heaven's Feel progressed since as the Servants were being defeated, her functions as a human being had to be suppressed so her body could act as a Holy Grail. However, due Kiritsugu hiding Avalon inside of her, it was possible to preserve her human functions to an extent by remaining at Saber’s side. She never told Saber any of this. But even Avalon's protection had its limitations and eventually she became physically incapacitated and was unable to do anything but to stay laying inside a magic circle and talk. When, Kiritsugu came to say his goodbyes before she turned incapable of remaining conscious, she entrusted Avalon to him. During the final days of the war, Kiritsugu and Saber searched the remaining Servants and Masters while Maiya was left to protect Irisviel. They were confronted by Berserker, in the guise of Rider, who then abducted Irisviel and mortally wounded Maiya. Following her abduction, she was taken to Kotomine Kirei, who asked her what Kiritsugu's wish for the Holy Grail was due his personal interest for the man. Dissatisfied with her answer that Kiritsugu wanted to create a world without war and that Kotomine would never understand her husband, she was knocked unconscious. With the war drawing to a close, Irisviel remained unconscious and dreamed about her daughter asking if her father will return. She realized then that the Holy Grail War was ending and that her consciousness had merged with the Holy Grail; the tainted artifact took on Irisviel's personality. When Kiritsugu was submerged in the contents of the Grail during his battle with Kotomine, he encountered the spirit of the Grail in the form of Irisviel, who went on to show him how his wish would be granted by the tainted Grail. After seeing all his loved ones destroyed in order to fulfill his wish, Kiritsugu realized his true wish was to revive Irisviel and be with Ilya, but found out he could not accept the Grail as a result. Destroying the illusions of his wife and daughter, he woke up from the vision presented by the Holy Grail and decided to destroy the Grail.