Uryuu, Ryuunosuke

The master of Caster, Ryuunosuke is a serial killer who summons Caster after murdering a family and using their blood as a component in the summoning ritual. 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. Ryuunosuke 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 to relieve boredom.

(source: Type-Moon Wiki)