Uryuu Ryuunosuke

The Master of Caster in the Fourth Holy Grail War.
Cheerful and openhearted. Despite easily reading the subtleties of other people's emotions, he doesn't care for minor details and is always positive and upbeat. He is a pleasant young man who doesn't shrink back from failure, and whose natural sense of adventure and curiosity push him to enjoy life to the fullest. Unfortunately, his only interests are torture, murder, and disposing of bodies.
This proclivity wasn't caused by a childhood trauma that warped his personality, or anything like that. He was simply born evil, just like Kotomine Kirei. However, unlike Kirei he was never indoctrinated by a preexisting system of morality, and so was never burdened by any sort of hesitation or doubt when it came to embracing his desires.
Though he had already killed 42 people before he summoned Caster, he had never been considered as a suspect even once—the police investigation was a total dead end, failing to turn up the evidence required to even open a case against him. Despite being a totally unmethodical pleasure killer, his ability to destroy evidence and hamper criminal investigations is almost transcendent. If he had decided to work as a professional assassin it's quite likely that he would have quickly become a legend within the industry. However, he's the type of person that draws a strict line between business and pleasure, so the idea of making money off of killing rubs him the wrong way. He never even stole anything from any of his victims, instead choosing to support himself by working normal, unassuming part time jobs.
He is fond of animals, and loves going to safari parks and watching nature documentaries on TV. However, it seems he can only appreciate violence when the target is a human. While he may admire sharks and other predators, he has never once desired to be one himself, because as far as he's concerned the most skillful hunter of humans is another human.

Fate/Zero material: Fate/Zero Encylopedia