Assassin (Fate/Zero)

Assassin is the Servant of Kotomine Kirei in the 4th Holy Grail War. His unique trait is the capability to manifest as multiple entities at once.

Appearance
The multiple bodies of Assassin can vary widely in age, gender, build, and style of dress, but in general, their hair appears purple, and their skin and clothing are nearly black. They wear the standard skull mask associated with the known Hassan of the other Grail Wars. While the female Assassin is given a design in Fate/Zero material, she doesn't actually have a face under her mask, much like True Assassin.

Identity
As with every legally summoned Assassin per the original, uncorrupted rules of Fuyuki Grail War, his identity is that of the legendary Persian "Old Man of the Mountain", Hassan-i-Sabah, the leader of al-Assasīn. Despite his possession of female personalities, his original existence is referred to with a masculine pronoun in quotation marks, which has been interpreted to suggest that he may have originally been physically male, at least. His wish to the Grail was ultimately "to be reconstituted as a complete personality."

Personality
Each of the Assassins has their own personality, and their experiences are not shared between them. The female Assassin is shown to be the main one to interact with Kirei. The Einzbern Consolation Room introduces the first Assassin to be killed by Gilgamesh as Zayd (ザイード). He has a polite tone of speech and a habit of pushing his mask up repeatedly while he speaks. He is very passionate and emotional, and he is upset that his role was to simply die. He is eventually convinced to have pride in his role, and vows to become Zayd, the King of Assassins, before dying in despair while seeing the others being slaughtered by Rider.

Fate/Zero
The Assassins initially act as reconnaissance agents for Tokiomi and Kirei, after Gilgamesh kills one of them in full view of the other Masters who possessed no specialization in any particular skill. This tactic successfully lead most of the Masters to believe that Kirei and Assassin had been defeated, and allowed the dozens of remaining Assassins to move freely without suspicion. Their continued existence, however, is discovered when Rider and Waver Velvet accidentally encounter three Assassins in Caster's lair.

The Assassins meet their end when they are sent to gauge Iskander's power levels. He complies by using his most powerful Noble Phantasm, "Ionioi Hetairoi: Army of the King", to massacre each and every one of the Assassins.

Urobuchi originally designed a plot line involving one of Assassin's personalities, a small girl, losing her memory and believing herself to be a normal child. Waver and Rider would have found her inside Caster's lair, believing her to be the only surviving victim. She would have eventually recovered her memory with Kotomine's use of the Command Spell, resulting in Waver having to make a difficult decision.

Abilities
Unlike True Assassin, who was one entity, the Zabaniya of the Assassin of the 4th War permitted him to divide into up to 80 separate existences. This was not a result of modifying his flesh in the manner of the 18 Hassans that predated him. Rather, the multiple bodies he possessed as a servant came of intentionally fragmenting his psyche in life -- a sort of self-inflicted dissociative identity disorder intended to compartmentalize the various skill sets and knowledge he had accumulated during his lifetime; a total of 32 distinct specializations. Summoned by the Grail, he obtained the capacity to express each personality as a independent body, and to relegate individual tasks to the personalities best suited to them.

As the nature of the Assassin's Noble Phantasm is "division," the sum of his iterations did not exceed his original existence in strength or capacity; that is to say, each independent fragment was far weaker than his original whole -- and owing to the generally weak starting attributes of his class, he definitively became the weakest Servant of the 4th War. However, the class ability of Presence Concealment was shared across all bodies without degradation, effectively permitting him to serve as an "intelligence network" in his multiplicity.