class: True Assassin (also translated as Sin Assassin)real name: Hassan ben Sabbah
measurements: 215cm (height), 62kg (weight).
colour: gray.
likes: difficult to tell since he got part of Assassin and Lancer's personality, though the real Hassan was keen on religion and "inner" experiences.
dislikes: cant' tell much, but if we consider the real Hassan we may say disobedience and betrayal.
master: Matou Zouken.
weapon: His modified arm and knives.
standing: Lawful evil.
noble phantasms: Zabaniya - Delusional Heartbeat (casts a curse on its target, creating a duplicate of him/her and by destroing it affecting the original being with death as well).
personality: Not much can be told about True Assassin since he's a very minor character in the third scenario. He's the silent type though, someone who barely speaks and just act, following orders and proceding with method.
spoilers warning!!!
personal history: True Assassin is summoned by Matou Zouken from the defeated Assassin. He's featured only in the third scenario as an effective replacement for Assassin. Being summoned incompletely (with a partial intelligence and personality) thus he needs to complete itself by assimilating Lancer too. He initially supports Sakura in her shadow form and he fights alongside her, following the orders of his master, who's only willing to take advantage of the situation and wait till the end of the Grail war. He's eventually defeated by Kotomine Kirei as he and Shirou are trying to save Ilya from the Shadow. We don't really know his plans and the reason why he joined the war since they're not explained in the storyline.
the legend behind: Hassan ben Sabbah isn't a fictional character and his story is quite well illustrated by several documents. He was a charismatic leader, a strong man who was able to create an organization as the Assassins, a sort of political and religious group which was foudned by him in Persia, in 1090. The followers of the organization promised absolute submission and loyalty to the founder who also had rights of death and life on everyone who was following him. They werent able to consider less fanatic mussulmans and that's why they were used to punish them with destructions and slaughters. The assassins where worshipping an entithy/divinity called Baffometto, which actually was the Holy Grail and it appears they left it to the Templars as they were leaving for France (that's why the Grail is considered as part of their well-known treasure). The name of the organization and of its followers comes from "Hachisch", the drug used among its members to reach a "superior" consciousness, abandon fear and get the courage to face death and the mission they were appointed to. The organization knew its decadence after the death of its founder happened in 1124.