Grandfather of Shinji and Sakura. Ancient magus, living for 500 years so far. Evil mastermind who wants the Grail at all costs.
Formerly known as Zouken Makiri, he was about 300 years old when he co-founded the Holy Grail War along with Nagato Tohsaka and Justeaze Lizrich von Einzbern. Zouken would create the Master and Servant system for the war, Nagato would make Fuyuki City the battle area for it and Justeaze would develop the Heaven's Feel system, create homunculi that served as vessels for the Grail and make her body into the Great Grail's magic circuits.
Deeply admiring Justeaze, Zouken was devastated to see her sacrifice and sought to stay alive for as long as possible to see her sacrifice result in the Grail's completion. However as time went on, frustration from the Grail being incomplete and his deteriorating body left Zouken to forget the purpose of why he lived for so long and developed an obsession with obtaining immortality. He then changed his family name to Matou.
To conceal his status as the Matou family's ancestor over the past 200 years, Zouken masqueraded as a distant relative of the family. However, those among the Matous became suspicious of his true age when they would discover multiple instances of the name Zouken among the family registry.
Zouken stands at 145 cm (roughly 58 inches) having no hair (blue in his younger days) and white eyes. He is normally dressed in green and black kimono.
Zouken is a shrewd old man with plenty of knowledge on magecraft to the several centuries he has been alive. He is shown to be strict and harsh in the training of magi within his family and is becoming increasingly frustrated with the thinning bloodline of the Matou family preventing the birth of any natural-born magi.
Lacking a magus within the Matou family to be his heir, Zouken would adopt Sakura Tohsaka a year before the start of the 4th Holy Grail War. Kariya, a childhood friend of Sakura's mother Aoi and Zouken's nephew, got word of this development and immediately returned to the Matou household in disgust with Zouken as he learns that she is already being put through the family's brutal training. Zouken would berate Kariya for leaving the Matous ten years earlier, seeing him responsible for further contributing to the decaying bloodline of the clan as Kariya was one of the few natural-born Matous born with magic potential.
Kariya would strike up a deal with Zouken to allow him to compete in the Grail War on the Matou family's behalf in exchange for Sakura's freedom should he win it. Zouken consents to the deal, but makes Kariya become infused with Crest Worms to amplify his mana output due to his lack of formal training as a magus. Secretly though, Zouken doubts Kariya's chances of victory in the Grail War, only accepting their deal on a whim in the event Kariya surpasses his expectations.
Towards the end of the war, Zouken would observe Kariya abducting Irisviel and handing her over to Kotomine Kirei. Zouken would confront Kirei and remark on his nephew having zero chance of victory in the Grail War, as well as commenting on Kirei having the same scent as himself. This comment leads the priest to toss several Black Keys at Zouken, yet the old man shrugs off the damage from the attacks. He then teleports away in laughter.
After the end of the Grail War, Zouken gathers leftover fragments from the Grail and infuses them into Crest Worms that he then puts into Sakura's body. Zouken plans to make her body into another Lesser Grail.
Zouken would not make an appearance in the Fate or Unlimited Blade Works routes.
In the Heaven's Feel route, Zouken becomes disgusted with Shinji's repeated failures in the Grail War after losing control of Rider. He decides to take matters into his own hands by summoning True Assassin from Assassin's remains and a corrupted Sakura (under the influence of Angra Mainyu) to eliminate the remaining Masters and Servants.
However, Sakura would eventually break free from Zouken's control after giving into her dark side. He is also killed through a combination of Kirei performing an exorcism on his body and Sakura destroying what was left of his heart. However,
Zouken still managed to cling to life as one of the worms that formed his body still lived. He tries to reach the Grail until he encounters Illyasviel wearing the Dress of Heaven. This sight restores Zouken's memory of Justeaze, whom he seen wearing the dress centuries earlier, as the old man finally gives in to death.
Zouken is a powerful magus with a large amount of knowledge on magic thanks to being alive for centuries. His body is entirely made out of a species of magical worms called Crest Worms which he used to prolong his life. With his body as it is, Zouken can't be killed through normal physical means as he can shrug off fatal wounds and reform his body by feasting on the flesh and blood of humans. He can also make use of worms as familiars that are capable of feasting on the magical energy of their human hosts, even if they are no longer alive.