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Kariya serves as the Master of Berserker throughout the length of the 4th Holy Grail War. He returned to the Matou family to serve as a representative of the family in the hopes that he could free Sakura, the daughter of his unrequited love Aoi, from the abuse she endured from them. The head of the Matous, Zouken, was left with no choice but to allow Kariya's participation since there were no other natural-born magus within the Matou family. But due to Kariya's lack of formal train as a magus, Zouken suggested that he be implanted with Crest Worms, magic worms capable of greatly augmenting the magical output of a magus at the cost of one's health as the worms slowly eat away at the insides of their host. Implanting the worms caused Kariya's appearance to change as his hair turned white and one of his eyes turn blue, as well as suffer from great pain that increases throughout the Grail War due to both the worms eating away at him and Berserker's participation in battles.
Because of the abuse Sakura has dealt with, Kariya has a strong hatred for her biological father, Tokiomi, who had her adopted by the Matous. As a result, Kariya took every opportunity he had to get revenge on him from having Berserker engage Tokiomi's Servant, Gilgamesh, in battle to trying to kill Tokiomi personally during the battle with Caster. The latter leads Kariya to nearly be killed by Tokiomi, until he is healed by Kotomine Kirei. The priest offers Kariya victory and the Holy Grail in return for the capture of Irisviel. Kariya resorts to using two of his Command Spells to have Berserker take on the form of Iskander in the abduction and Kirei re-supplies his Spells through whatever spares received from his father.
Kariya is informed of the location of Tokiomi's home, under the belief this would be an opportunity to exact his revenge. Unbeknownst to Kariya though, he had been manipulated by Kirei for the priest's personal gain and he had already killed Tokiomi earlier on. Upon discovering Tokiomi's corpse, a surprised Kariya gets further shock when Aoi walks in on the encounter and assumes Kariya had murdered her husband. In her grief, Aoi accused Kariya of not caring for anyone and being responsible for taking away her husband and daughter. This leads Kariya into a blind rage and causes him to lash out at Aoi by strangling her to death. Upon doing this act, Kariya suffers a psychological breakdown from what he had done. He eventually succumbs to death when he can no longer supply Berserker's massive mana use for battle and he dies in sight of Sakura at the Matou estate, happy with dreams of living as a family with himself, Aoi, Rin
AbilitiesWhile lacking training and experience as a magus, the Crest Worms in Kariya's body allow him to supply the high mana costs for Berserker at the cost of his health and give him the ability to control Blade Wing Worms, an evolved form of the Crest Worms that are carnivorous and eat human flesh.