Hundred-Faced Hassan

The Heroic Spirit “Assassin” that participated in the Fourth Holy Grail War. One of the many “Old Men of the Mountain” that succeeded the name Hassan-i Sabbah.
Unlike the other Hassans, his “Zabaniya” was not the result of body modification. Instead, he took advantage of his inherent multiple personality disorder to operate as an unidentifiable assassin.
During the process of being summoned as a Servant, all the fractured pieces of his soul were materialized as separate individuals. However, because he still only has the spiritual potential of a single person, each individual Assassin has extremely low parameters—the lowest a Servant can have, in fact.
Also, each personality has its own strong and weak points. Amongst their numbers, there are even morons that disregard orders in their haste for glory, and klutzes that suck at surprise attacks despite being assassins. With a proper understanding of their personalities and idiosyncrasies, and careful planning to make sure each personality was at the right place at the right time, they could have posed a formidable threat. Unfortunately, his Master had no intention of seriously trying to win the war, so he was relegated to the role of a mere voyeur.
Though he was summoned by a Kotomine Kirei that was still Tohsaka Tokiomi's lapdog, and had resigned himself to the unrewarding duties of the servant of a servant, he too had a secret wish that he wanted to be granted by the Holy Grail—namely, the acquisition of a unified, perfect personality. If he had the chance, he had intended to betray Kotomine and Tokiomi at the climax of the battle and steal the Grail for himself. But sadly, as the Fourth Holy Grail War's very first casualty he never had time to reveal his rebellious spirit.

Fate/Zero material: Fate/Zero Encylopedia