The Master of Assassin was a relative of Faldeus who participated in the Third Holy Grail War. Summoning Assassin, he made use of his Servant by having Assassin mix in with his dolls for great efficiency in tricking opponents into believing he was a lone Master. He ultimately lost, and the Grail was unable to be utilized.
After the Holy Grail War, his memories of the events of the war were passed down in a doll. Those of Faldeus' clan were uninterested in the memories until Faldeus' grandfather proposed the idea to start the False Holy Grail War in the United States using the memories as a basis.
During Avenger's partial recreation of the Third Holy Grail War, Bazett Fraga McRemitz encounters the Master of Assassin outside of Homurahara Academy. Bazett is able to overwhelm his dolls due to her fighting experience, but Assassin surprises and kills her with Zabaniya: Cyber Phantasy.
The Master of Assassin is a skilled doll user, able to control a large amount of French dolls as if they were his own limbs. They are equipped with precise killing mechanisms, which are a threat even to Bazett Fraga McRemitz. During the war, Assassin, a small figure, poses as a doll wearing a skull mask, surprising enemy Masters.