Justeaze has a doll-like face considered to be "too beautiful and too well-crafted", red pupils likened to rubies, and a "bell-like, beautiful voice." All those modeled after her share the same appearance, with Kirei Kotomine considering Irisviel von Einzbern to look exactly like her portrait.
Through Illya, Justeaze displays a cold and aloof personality, calmly chastising Zouken over how he has lost his way from their original desire of the ascension of humanity to living only for the sake of obtaining immortality.
Justeaze was the head of the Einzbern family two hundred years ago. Well acquainted with Zouken Matou, they had a dream of reaching a utopia that could eliminate and cure all hatred and suffering. Though they wished to exceed the limits of humanity by surpassing the body and reaching the soul, they were particular about reaching that state while still having human form. They were also acquainted with Nagato Tohsaka and Kishua Zelretch Schweinorg, considered a potential inheritor of the Jeweled Sword of Zelretch if not for Zelretch appraising the family as fundamentally evil. Along with the family's then eight hundred year wish of retrieving their lost Third Magic, they sought to use the Holy Grail to reach that miracle. She was born to become the core of the Greater Grail, and she was the one to devise the ritual. Accompanied by Nagato and Zouken, who provided necessary land and starting materials for the ritual, and being watched over by Kishua Zelretch Schweinorg, she was sacrificed as the key to reenacting the ancient sorcery.
The Einzberns passed down a portrait of her throughout the ages, and as of the Fourth Holy Grail War, placed a stained window in the ceremony chamber of the Einzbern Castle depicting her with Nagato and Zouken, all reaching their hands to the grail in the sky. It was made to depict the Tohsaka and Matou families waiting on her to show how much they look down on the other families and the humiliation felt in having had to rely on them. Referred to as the "Primeval Justeaze", all subsequent homunculi of the Einzberns were manufactured to the standard of her make and used her as their model. Within their hearts were contained each previous model, a record of their experiences that can be traced back to her. This allowed all those modeled after her to feel her "pain." Illyasviel von Einzbern was developed as one that could be considered her "latest model", born from a "Justeaze-type homunculus" and a foreign magus.
Justeaze still exists as the core of the Greater Grail as of the Fifth Holy Grail War, but will eventually be destroyed during the events of the war during the Heaven's Feel route or dismantled ten years later in a conflict meant to end the war forever. When Illyasviel von Einzbern first confronts Zouken during Heaven's Feel, her personality is briefly brought to the forefront when Zouken rants about his reason for wanting immortality. She remarks that he has fallen, reminding him of their shared desire. He laughs at the mimicry and prepares to take Illya for the purpose of obtaining the Einzbern's grail. Illya later accesses her memory and shares it with Shirou Emiya to reproduce the Jewel Sword. It is the memory before she became the core, looking upon her compatriots as she sits in the center of the labyrinth of circuits before the ritual begins.
With the later collapse of the cavern as the last remaining part of Zouken finally begins to die, Illya, planning to sacrifice herself to close the gate to the other side and save Shirou, appears before Zouken and speaks as Justeaze. Zouken does not even see Illya, only seeing his partner who sacrificed herself to create the Grail. She only says "I ask of you, my foe. Why do you not want to die?", reminding Zouken of their dream. He tells her that she is right as he realized what he had truly wished, accepts that all is out of his reach, and passes away.
Justeaze is a archmagus who, as a homunculus, was born as a living Magic Circuit for the sole purpose of becoming the core of the Grail. She is known as the "Master of Northern Alchemy", and Kiritsugu Emiya appraises that none of her descendants can be overcome in the process of making superb homunculi. She is responsible for devising the Heaven's Feel system, and still functions in the center of the Greater Grail. Her will is needed for the system to function, else the "Holy Grail is just a Holy Grail" without her. The circuitry covering the cavern in which it is situated is her magic circuits, enlarged and multiplied to reach nearly a size of a kilometer in diameter, acting as a "microcosm of the universe created from a single human body." Due to the her model being imprinted upon the subsequent Einzbern homunculi, they share her memories, and Illyasviel von Einzbern displays brief traces of her personality that surface around Zouken.