Assassin's True Name is Hassan-i-Sabbah, given the moniker of Hassan of Intoxicated Smoke (煙酔のハサン, Kemuriyoi no Hasan?), one of the nineteen leaders of the Hashshashin to take the name over many generations.
In game, Assassin only appears briefly with art assets shared with Hassan of the Hundred Faces, so it is unknown if that matches Assassin's true appearance.
Assassin, working with other Hassans like Hassan of the Cursed Arm, is helping to protect the citizens of the Holy Land when confronted by Tristan. Unable to save them and herself, she proposes giving up her own life for Tristan not to take a single step towards the refugees upon his chivalry. Killing herself, Tristan still kills all of them, claiming that she should have told him not to move a finger. She becomes a Shadow Servant after Tristan leaves, rising with the enraged spirits of the refugees, and fights the Protagonist. After being defeated, she begs for the protagonist to give her comrades a proper burial before fading away,
Hassan of the Cursed Arm describes Assassin as a master of assassination among the Hassans gathered in Camelot.