He was summoned by Caster as a guardian and watchmen for the front gate of the Ryuudou Temple. As an improper servant, Assassin is entirely dependent on Caster for mana and cannot move freely beyond the grounds of the Ryuudou Temple. Furthermore, he lacks most of the usual parameters and abilities granted to his class. Because of his limited status, Assassin cares little for the Holy Grail, and outside of his duties to Caster, he only seeks to enjoy a decent sword fight.
His real identity is Koujirou Sasaki, the famous Japanese swordsman.