Master of Berserker during the 4th Grail War. For the sake of his unrequited love and to save Sakura Matou, he undergoes intensive and cruel magical training which costs most of his health and sanity.
Kariya is the uncle of Shinji and younger brother of Byakuya for the Matou family. He originally severed ties with the Matou family for ten years out of disgust for the family's cruelty. Without his family's resources, he took up a profession as a freelance reporter.
While still with the Matous, Kariya befriended and fallen in love with a young woman named Aoi, whose familial ties to the Zenjou family granted her potential to give birth to a powerful magus and the Matou family head, Zouken, had arranged for marriage to obtain that heir. However, finding out of Zouken's plans for his heir led Kariya to sever ties with the Matous and sever his arranged marriage to Aoi.
Kariya was originally created for the Fate/ Zero novel series written by Gen Urobuchi. His character design originally had a more disfigured appearance that would make him seem more villainous when corrupted by Zouken's Crest Worms for the novels. However, the design was revised in the final version of his design.
Kariya stands at 173 centimeters (roughly '5'8") and weighing 55 kilograms (about 121 pounds). He has brown hair and gray eyes at the start of Fate/ Zero. However due to his body's corruption by Zouken's Crest Worms, his hair and eye color changed with his hair turning white and his left eye turning light blue.
Kariya started off as a kind man who left his family out of disgust for their inhumane and cruel actions. He had a strong love for Aoi and believed that he was under great care from her marriage to Tokiomi. However as his mind became warped due to his body being corrupted by the Crest Worms, Kariya becomes gradually incapable of comprehending complex thought and his aggression is greatly heightened. This makes him easy to be manipulated in the later half of the Grail War and affect his actions toward Tokiomi. While at first desiring to defeat Aoi's husband in the Grail War, his mind becomes warped enough as time wears on to the point where his thoughts change towards wanting to kill Tokiomi. He is unable to comprehend that Aoi cares greatly for her husband and doesn't want neither Tokiomi nor Kariya to fight one another.
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 until she passes out from the act. Upon doing this, 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 and Sakura.
While 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.