Jubastacheit is a golem made to control the Einzbern Castle. Declared the eighth head of the Einzbern family, Jubstacheit was not around during Justeaze Lizrich von Einzbern's era, but kept himself alive for nearly two centuries by continuously prolonging his life. Since the Second Holy Grail War, he led the family in their attempts to reclaim their lost Third Magic, but suffered failure twice. During the Third Holy Grail War, he suffered defeat on two fronts, with the family's Servant, Avenger, being quickly defeated and the destruction of the Lesser Grail. With the Grail being unable to manifest even before the victor emerged, the war became meaningless. He reflected on the design and decided to "cover" the vessel of the Grail in a humanoid shape in order for it to be able to self-manage. As the he and the elders of the family sought victory over all else, he hired Kiritsugu Emiya to act as their representative to make up for their weakness in combat during the previous rituals, and sought out a relic to summon the "strongest Heroic Spirit of the Sword."
Jubstacheit is the "old magus king" of the Einzbern castle. While long lived, Kiritsugu feels an "astute light" emitting from his eyes to hide any trace of senility, and his white beard makes one think of a frozen waterfall. He gives off an imposing bias and pressure each time Kiritsugu catches his gaze. He has a close relationship with the Talbot farm that produces early autumn Darjeeling, and he has served the tea only to special guests for a number of years. Kiritsugu shows him respect, but they have fought over details such as installing a telephone line and power generator, which is heresy to such an established magus.
Since taking the family under his leadership, they have changed their goals from "seeking the Holy Grail" to "winning the Grail War." Much like the rest of the family, he wishes to reobtain the Third Magic, but he has only known the pain of defeat in his efforts to reclaim it since the time of the Second Holy Grail War. After suffering failure during the Second and Third Grail Wars, the anxiety of facing the third opportunity during the Fourth War is extraordinary. All of his emotions can be condensed into the single word "Achievement", and Kiritsugu believes his spirit likely only has this rigidity towards achievement left.
Fate/Zero Drama CD
The product of Jubstacheit's goal of a humanoid vessel for the Grail is the Homunculus, Irisviel von Einzbern, nine years before the Fourth War. He creates her with the purpose of being a vessel capable of self-defense, but Kiritsugu finds her to be lacking. He wishes for something without a sense of self because an object that can decide on its own will is no longer a tool. Upon Kiritsugu complaining and asking for a more compact animal form, Jubstacheit reveals that he has thrown her naked into the homunculi disposal yard as a test, planning to see if she can survive for days in the forest with hungry wolves and evil spirits. Kiritsugu is astonished, but Jubstacheit states that all he needs is a vessel that will survive until the Greater Grail ritual. If she does not survive, he will simply acknowledge the defects in his design and prepare a new vessel in accordance with Kiritsugu's specifications. Rather than let it play out, Kiritsugu quickly rescues her, noting the entire ordeal was to clear the humiliation from Kiritsugu hurting his pride.
Jubstacheit summons Kiritsugu and Irisviel before the advent of the Fourth Holy Grail War to present them with Avalon, the relic to summon King Arthur that they had requested to be excavated from Cornwall. He confirms the status of the vessel of the Grail with Irisviel, and his eyes shining with "insane strength" declares, "This time... no one must survive. Kill the six Servants; this time we must acquire the Third Magic, the Golden Grail." Although both Kiritsugu and Irisviel answer him fervently, Kiritsugu's true mission lies elsewhere in obtaining his dream of world peace.
The ritual ends in failure, and he learns of Kiritsugu's betrayal in the destruction of the Holy Grail. Rather than seeking to punish he who would have been unable to do anything but silently accept it, he instead never again allows Kiritsugu into their forest's Bounded Field and keeps him away from his daughter, Illyasviel von Einzbern. Kiritsugu believes that he either wishes to exile him to carry the shameful title of a traitor for the rest of his life, or to impose the most severe punishment that can be inflicted by never allowing him to see his daughter again.
Jubstacheit is not mentioned by name, but Ilya, referring to him as grandfather, mentions being taught details about the Holy Grail War by him. He described Japanese people to her as a "neck-cutting tribe who die by having their necks cut off while sitting Japanese-style." Another mission that she is given is to slay both Kiritsugu and his adopted son, Shirou Emiya.
Jubstacheit is skilled in making homunculi, and claims that Irisviel is among the best of his works in recent years. She is constructed to be harder to break than a regular Grail vessel, and designed to prevent herself from being broken. He cares little for the well-being of his creations, prioritizing his pride as a designer over any pain they may experience. He has used an unknown method to prolong his life for two hundred years.
 Fate/Zero Season 1 Boxset - Drama CD 1