The Servant of Kirei Kotomine during the 4th Holy Grail War. Assassin has the unique ability to manifest as multiple entities at once.
Like other Assassins legally summoned in past Grail Wars, the identity of this one is Hassan-i-Sabah, a legendary Persian leader of a group of assassins called al-Assassin who is known as the "Old Man of the Mountain". Each Assassin in Hassan's form summoned in the various Grail Wars are a different personification of the Old Man seeking the Grail to "be reconstituted as a complete personality".
The female Assassin among the group was given a maskless facial design in character material books made for Fate/Zero. The series writer, Gen Urobuchi, admitted he might have winded up making some story focus for the female Assassin if he was aware of her appearance, as he admired her character's muscular appearance. Much like True Assassin during the events of Heaven's Feel in Fate/Stay Night though, the female Assassin lacks a face beneath her mask.
Urobuchi also originally had plans to have a child personality of Assassin interact with Waver and Rider to lighten the story of the series up around the second and third volumes to Fate/Zero. However, the idea was scrapped to avoid giving readers confusion over who was the main character of the novel. This storyline would have involved Waver and Rider encountering the girl in Caster's lair with Waver assuming her to be one of the children he abducted, unaware she was the last of the Assassins still alive following nearly the whole group being slain by Rider who was suffering from amnesia. Rider was aware of this fact of her true identity, yet allowed Waver to still care for her up until she regained her memories via a Command Spell used by Kirei to have her attack the Einzbern Castle. This would result in tragic events that would shape the growth of Waver's character for the series.
Assassin would appear in the anime adaptation of Fate/Zero made by Ufotable. In Japanese, the Assassins were voiced by Abe Akina, Takuo Kawamura and Eiichiro Tokumoto. In English, they were voiced by Karen Strassman, Patrick Seitz and Vic Mignogna.
Each Assassin appears in differing types of clothing and sizes. But they share common features in that their hair color is purple and that their skin color and clothing are black.
Each of the different Assassins have a different personality and role to be fulfilled, though they all share the same goal of wanting a complete personality through the Holy Grail. The female Assassin seen regularly interacts with their Master, Kotomine Kirei, and the first Assassin killed by Gilgamesh, Zayd, had a formal tone with his speech, being passionate and emotional. They were all distraught at the fact that Kirei had no intention of winning the Grail War due to lowering himself to be Tokiomi's servant, thus they had conspired plans of their own to seek the Grail which they couldn't act on due to being the first Servant slain in the Grail War.
The Assassins were summoned to serve as reconnaissance for Kirei and Tokiomi. When Zayd was slain, the other Masters and Servants assumed that Kirei was out of the Grail War, unaware of the many other bodies that formed the Heroic Spirit. Iskander and Waver would learn of the deception while traveling through Caster's lair and seeing three of the bodies. They would all be killed off later on when Iskander summons his armies through Ionioi Hetairoi to slay the group.
Unlike prior Assassins summoned in the Grail War, the one summoned in the 4th Holy Grail War consisted of 80 different existences that contain the differing personalities, skills and life experiences that Hassan carried with him in his lifetime while alive. This resulted from a self-induced dissociative identity disorder Hassan had to divide up the various skills and experiences he accumulated while alive. Because of this "division" status, each of the Assassins are far weaker than their combined entity, making them the weakest Assassin summoned on record in the Grail War. However, all the Assassins retain the Presence Concealment ability of their class, allowing their presence to remain undetected to any Servant unless they are visible to be seen.