Ryuunosuke Uryuu

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

Character
Uryuu Ryuunosuke is a serial killer. Though he is not a mage, 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.

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, Ryūnosuke 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, Tohsaka Tokiomi and Kotomine Risei soon see Ryūnosuke and Caster as obstacles in their plan to obtain the Holy Grail.

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

Servant
Uryuu Ryuunosuke's Servant is Bluebeard, a Caster-class Servant, who was summoned using the blood of a murdered family possessing magical circuits. Caster is primarily interested in Saber and appears to be familiar with her tragic demise.