One of the nineteen leaders of the Hashshashin sect and the 19th who took the name of Hassan-i-Sabah and the mantle of Old man of the Mountain. This particular individual suffered from Multiple Personality Disorder which he utilized to compartmentalize his abilities. This gave his Noble Phantasm the ability to manifest his different personalities as separate and autonomous entities. Though they differ in terms of build and sex, the different personas share similar characteristics: they have dark brown skin, purple hair (for those who have it) and wear white skeletal masks. Like all other Hassan-i-Sabah, he can be summoned only in the Assassin Class with the class itself acting as a catalyst. Personas • Asako (アサ 子) • Big Assassin (でかアサシン) • Child Assassin (ちびアサシン) • Macour the Quick (迅速のマクール) • Strange Arm Gozour (怪腕のゴズール) • Zayd the Base (基底のザイード) • more... In Fate/Zero

Spoiler

Assassin serves as the Servant of Kirei Kotomine in the 4th Holy Grail War being mainly occupied with reconnaissance duties. The main personality that interfaces with Kirei is a young woman by the name of Asako.