A serial killer and the Master of Caster in the 4th Grail War. Ryūnosuke knows nothing about magic or the Holy Grail War; his involvement in the war is simply to watch Caster kill people in unique and thrilling ways.
Ryunosuke Uryuu is a serial killer who worked as a "freeter", someone who took simple work that would be resolved in hours and small pay but allowed more free time. Unknown to him, his family is descended from a line of magicians yet knowledge on this was lost among recent generations of his family. Desiring more creative ways to kill his victims, Uryuu came across a spellbook that he thought gave instructions on how to summon demons. Unaware to him though, the spellbook had instructions on how to summon a Servant for the Holy Grail War.
Uryuu's character was hard to grasp during development of the Fate/Zero novel. His general design was the same throughout all stages of development, but his facial expressions were different than earlier designs of his character with Gen Urobuchi originally thinking of him as having a "toned-down madness" compared to Caster before not knowing how to think of him. For the anime version of Fate/ Zero, Uryuu was voiced by Akira Ishida in the Japanese version and Johnny Yong Bosch in the English dub version.
Uryuu stands at 174 centimeters (roughly 68.5 inches) having orange hair and brown eyes. He is seen dressed in purple jeans and a shirt throughout the series.
Uryuu has a simple personality fascinated with death and dying, thinking that humanity is curious in death like it is something forbidden. He is a malicious and emotional man thrilled with the enjoyment of brutal activity and he's known to murder women and children since they are easy targets for him. Before meeting Caster, he would break into the houses of victims he wanted to murder. With Caster in his life, Uryuu gained a monster whom he could witness inflict as many creative deaths as he would enjoy and the Servant's ability to keep human victims alive via his magic allowed Uryuu to experiment in ways he could torture his victims such as turning them into living instruments or furniture.
Using the spell book he found in his parents' basement, Uryuu slaughtered several families and used the blood of his victims to summon Caster, at first mistaking the Servant as a demon. While chosen as a Master for the Holy Grail War, Uryuu lacked a catalyst to summon a specific Servant thus the Grail chose a Heroic Spirit with the closest personality traits they had to him. Making use of a personal lair within the sewers of Fuyuki, the two psychopaths would abduct many children within the city to torture and kill for their own sadistic pleasures.
With the two having no regard for the rules of the Holy Grail War, the competition is temporarily suspended by Risei Kotomine who offers extra Command Spells to whomever can kill off Uryuu and Caster. Despite these events leading to Uryuu and Caster being hunted down by other Servants in the Grail War, they continue to murder children and cause fear to stir up throughout Fuyuki. For their final plan, Caster summons up a giant monster from his spellbook that he intends to unleash upon the populace of the city. This leads the other Servants in the Grail War to confront Caster with the battle being a difficult one thanks to his monster's constant regeneration. As Uryuu gets glee out of seeing these events unfold, he winds up being killed by sniper rifle shots coming from Kiritsugu Emiya.
While not recognized officially as a magus, Uryuu was descended from a line of magi and this familial connection would allow him not only to summon Caster, but also for his Magic Circuits to awaken. He was given a magical bracelet by Caster that allowed him to hypnotize children into following his commands and leaving them in a trance, using this as his means of abducting them. The effects of the bracelet could be overcome by either destroying the bracelet or using one's own prana to counteract the effects of the hypnosis as Rin Tohsaka would perform during her encounter with Uryuu in the anime.