Ryuunosuke Uryuu

Ryuunosuke Uryuu (雨生龍之介) is the Master of a Caster class servant in the 4th Holy Grail War.

Background
Uryuu Ryuunosuke is a serial killer. Though he is not a magus, he was selected by the Holy Grail to fulfill the war's requirement of seven Masters and seven Servants. Ryūnosuke knows nothing about magic or the fourth Holy Grail War; his involvement in the war is simply to watch Caster kill people in unique and thrilling ways.

Personality
He is obsessed with the process of death and dying. He thinks that humans are inherently curious in death like a sort of forbidden knowledge and try to understand it by observing other people's death, and make up a virtual experience, so that's why he became a murderer. According to him, claims that depiction of cruelty in fiction has a bad influence on young people are ridiculous nonsense, because if blood and screams in splatter horror had been at least a little more realistic, he wouldn't have become a homicidal maniac. He is satisfied by squeezing the best out of the life force of those he kills, of feelings such as the attachment for human life, anger and affection. To let his victims know the exact time and circumstances of their death, that in itself had a deep meaning as rich as a miniaturized life and the more he killed people, the more he understood the lives he took. That knowledge, that discernment in itself was a kind of dignity, a style that in his distorted logic, he considered as something "cool". With his death, his last thoughts were the brightness and vibrance of his lifeblood and the excitement as he experienced his own death.

Role
While searching for different, more "amusing" ways to commit murder, he discovers a book at his parents' house with instructions on summoning a Servant. Shortly before the fourth Holy Grail War begins, Ryuunosuke murders an entire family with magical potential, with the exception of a child to feed the demon he intends to summon, and uses their blood to perform the summoning ritual outlined in the book. He successfully summons Caster.

As a result of the mass amount of senseless killing both Master and Servant perform, Tokiomi Tohsaka and Risei Kotomine soon see Ryuunosuke and Caster as obstacles in their plan to obtain the Holy Grail.

Ultimately, Ryuunosuke is sniped and killed by Kiritsugu Emiya as Ryuunosuke is observing a fight between Caster, Berserker, and Archer.

In the Anime adaptation of Fate/zero, he met and nearly came close to killing Rin Tohsaka.

Abilities
Uryuu Ryuunosuke is not a magus, but he is descended from a lineage that once resided in Fuyuki. Using notes left behind by his ancestors, he summons Caster by creating a summoning circle as outlined and using the blood of the murdered family as a catalyst. While forgotten by their descendants, their blood still carries their linage's "strange power." Uryuu's own Magic Circuits had remained sleeping until that day, which unleash within him like a tidal wave in order to call out to Caster. While he used an unskilled summoning circle and he didn't perform the necessary chant, such things are only normally done as a precaution by magi to create a bond with a perfect, reliable Servant. If a human has a proper catalyst in order to summon a Servant, the requirements of the Grail can be met even if quality of the ritual isn't great.

He is shown to use a bracelet created from Caster's magecraft that allows him to easily enchant children into obeying his commands. Simply touching a victim places them into a trance where they will freely follow him to his hideout. The victims will remain in that state unless the bracelet is destroyed, which immediately frees them from the trance. Rin Tohsaka is able to use her own prana to counteract its effects and overload it much like her own jewel magecraft.