Assassin's True Name is one of the nineteen Hassan-i-Sabbah able to be summoned under the Assassin class in regular conditions of the Fuyuki Holy Grail War system. The specific name of this Hassan is unknown.
Assassin is able to work in tandem with his Master, allowing him to successfully kill surprised opponents.
Assassin's activities during the Third Holy Grail War are unknown. During one loop of Fate/hollow ataraxia, Bazett Fraga McRemitz and Avenger encounter his Master in the forest behind the school. Although she assaults him with confidence, she is surprised by Assassin and quickly killed. Avenger is killed as well, and they plan for countermeasures for the next time. She believes luring him onto flat terrain would allow her to tack his movement enough to hit him.
Assassin is a midget whose height is no greater than that of an adult's knees. It may have been a characteristic he possessed in life, or it may be a special characteristic exaggerated by becoming a Servant. It does not match that of a dwarf in a fairy tale, but that of an acrobat cultivated over time. It would normally make him a worthless Heroic Spirit suited to be a clown in a circus, but it presents a strong advantage for one of the Assassin class. Detecting him is difficult, the areas available to hit are overwhelmingly small, and he is among the fastest of the Servants.
His Master is a skilled doll user who utilizes French dolls equipped with various killing mechanisms, controlling them as if they were his limbs. They are a threat even to someone like Bazett, but she is able to work through them with her experience. Assassin poses as one of the dolls, differentiated by his skull mask, and surprises opponents with poison needles able to immobilize struck body parts before assaulting them. Using his size, it is hard to track him with unmodified eyes, and he can use terrain to hide himself even more.
Although he has a weak body that cannot handle impacts well, he is swift enough to dodge dangerous attacks an instant before they hit, and he can maneuver himself onto the opponent's body. He would be weaker if fought on flat terrain, taking away some of the advantage of his size. Without anything between him and the opponent, simply touching their face is enough to unleash his one-hit Noble Phantasm, Zabaniya: Cyber Phantasy, killing the opponent by turning their brain into gunpowder and blowing them up from the inside.