Assassin is Sasaki Kojirou, a mythical Japanese swordsman who is said to have never existed. And he never did. This Sasaki Kojirou is just an individual with similar traits to the legendary swordsman. When he died, he died lacking a name, and he was never talked about or heard of again. Several centuries later, the Holy Grail War started, and seven mages each summoned a spirit from the past to fight for them in a tournament for the Holy Grail, which granted the winner one wish. The Servant Caster, a master mage herself, tried to summon the spirit of the legendary Sasaki Kojirou, but since he never actually existed, the nameless samurai was summoned instead, with the traits and personality of Kojirou as he was described in legend. Being summoned under the Assassin class as a false Servant, the new Kojirou was bound to Ryuudou Temple, which Caster used as her base, and was assigned to guard the temple's gate from intruders.
Vitrification: A skill that puts Assassin in a serene state of mind, hiding his presence and preventing mental interference.
Eye of the Mind (Fake): A special sixth sense that Assassin can use to detect danger and sense invisible objects.
Tsubame Gaeshi: The legendary signature technique of Sasaki Kojirou, which Assassin inherited after being summoned. This technique lets Assassin slash three times at once, and is completely unavoidable. It works by briefly bending the laws of physics to multiply Assassin's sword, letting him slash three times at once from three different angles.