Assassin's True Name has been forsaken as of before the time she gained the properties of a Heroic Spirit, but she is given the monikers of No Name Assassin and Beautiful Assassin (美しき暗殺者, Utsukushiki Asashin?). She was a former candidate for the position of Hassan-i Sabbah, the pseudonym shared by the leaders of the League of Assassins, the Hashshashin. She was known as a particularly faithful girl in a certain country whose existence is referred to as "merely a story." She underwent intense training to earn herself a name, and she was pious to the point in attempting to prove her faith that she was labelled as a "zealot" with despise by even those who worshipped the same God. While their group could be called a company of fanatics, her convictions were so beyond the norm that they earned her that title. She did not hate those who disparaged her because she believed the reason she was despised was because of her own immaturity and that her conviction was not firm.
She wished to become the sect’s leader, which required that she acquire power that could be called Divine Miracles. These Miracles had some unique specifications in that they must all quickly and reliably snuff out the lives of heretics and infidels. The leaders in the past obtained their titles by achieving numerous miracles worthy of bearing the name of the 19 angels that guard hell in the form of Zabaniya, so she continued her mad pursuit of self-improvement incessantly, seeking the miracles that were achieved by her predecessors. She recreated them one by one, but the World seemed to only shout “Still not enough” at her without pause. The other members were shocked by her achievements to the point where they could not believe what had happened. While she had achieved the miracles of all eighteen leaders in the past, no one in the sect acknowledged her right to inherit the leadership. It was her proof, soaked in blood and sweat, and she knew better than everyone what it really took to reach that point.
The faithfuls began to shun her, telling her that “What you achieved is no different from making copies of old tomes. You have not achieved your own Miracle, because there is still immaturity within you.” While she did lack the ability to innovate, their denial was only half of the reason. They feared her, she who learned all of those techniques in the span of a few years. Their final reasoning was that “You are still too young. We cannot have someone like you bearing the title of our leader.”, and although it was filled with nothing but excuses, she obeyed without question. She simply questioned her own faith, and believed herself to be juvenile in having defiled the secret techniques of their past leaders. She did not blame anyone, but still continued the quest for her own secret technique.
“The Hundred-Faced One”, a man capable of resolving every single situation masterfully, eventually became the nineteenth leader, and upon recognizing his ability, Zabaniya: Delusional Illusion, was one she did not have, she was ashamed of her own immaturity rather than filled with envy. She did not fulfill anything in the end, nor did she receive any acknowledgement. She merely lived as a zealot, and she was not even allowed to be martyred for her faith. She spent her life meaninglessly until the end, but still bore the World no hatred. Still ashamed of her immaturity, she again dove into the whirlpool of faith because she has no emotions to allow her to bear a grudge. The only things she despised were the false idols of infidels, making her a zealot that no normal man could redeem. That is her whole story which merely disappeared into the abyss of history as that of another zealot, and while it should have ended there, it was reborn the instant when the zealot become the False Grail’s chosen.
Assassin, having never earned the title of Hassan-i Sabbah, is different from the other nineteen candidates. They normally each have a white skull mask that hides their hollowed out face, but her own features are entirely intact. She does wear their usual black robe, but her face is obscured by a veil and not a mask, showing a pale skin complexion and golden eyes.
Assassin works entirely around the idea of faith, focusing on nothing else. She always sought to prove herself, to prove that she was faithful, and to prove that she was a true believer of God. She realized long ago that the goal she sought was childish, but still continues to follow it without faltering. She swallowed the pain in learning her techniques without regret, and she blames anything that impedes her path on her own shortcomings, whether it is actually her fault or not. Her only wish upon the Grail is to simply destroy it as a proof of heresy. While her summoning may be a prank played by Fate, the knowledge granted by the Holy Grail allowed her to instantly learn of her own change in Fate. While she seems to find purpose in eliminating the "heretical" magi that serve Jester, she seems to grant them a chance to escape and has a hint of sadness and sympathy before slaying them.
Part of the knowledge obtained from the Grail allowed her to learn that numerous past leaders have also sought the Holy Grail. While it saddens her that they tried to obtain it, she neither hates them or looks upon them with scorn. She still believes that they were more faithful than herself, and even now believes she should pay them respect. Her true hatred goes towards the very existence of the "Holy Grail Wars" for leading the previous leaders astray. She wishes to destroy it personally, even if she may disappear in mere days, hours, or seconds. She feels that even if her body is only a temporary illusion, so she has no doubt for her course of action. She believes in her own existence, and she believes she owes the faithfuls to do what she is about to do: wage war against the entire Holy Grail War.
Assassin is summoned by Jester Karture and ten of his disciples in the basement of a mansion near the Lake Region of Eastern Snowfield. In his attempt to obtain a Servant of the Assassin class, he summons a nameless Assassin rather than what he believed was going to be one of the nineteen regular Assassin candidates. Confused by her appearance, he observes her curiously while waiting for the question to fully complete their contract. Upon her asking if he is her Master, she instantly kills him by crushing his heart before he can even finish his confirmation. The disciples are outraged by their master's death, and ask why she killed him if she is a Servant. She only replies “Our God… has no Holy Grail…” in a wispy voice, while one man tries to attack her with a sword. She quickly kills him with before he has a chance to strike.
While she seems to wait for the other nine pupils to escape, they instead attempt to attack all at once with magecraft. They are left dead an instant later, while she silently climbs the stairs. Dematerializing her body, she dashes into the darkness of night in her invisible state. She seems aimless, but her goal in destroying the Holy Grail as heresy to her faith is clear. She would normally only have a limited time after slaying her Master, but Jester is still alive after having his heart crushed due to his nature as a Dead Apostle. After learning of her nature through the path of magical energy established between them, he abandons all of his previous goals, so that he can make her submit and savor her expression while crushing her faith and draining her strength.
Assassin sneaks in the opera house, only to find Cashura about to summon his servant. She used her Zabaniya to kill Cashura. Although Assassin coincidental saves Ayaka Sajyou, Assassin questions if Ayaka seeks the Holy Grail. Before Ayaka can respond, the summoning of Saber was called upon. Assassin engages Saber but she escapes as soon as Saber uses Excalibur.
As Ayaka and Saber were taken to the police headquarters, Assassin breaks in and tries to target Saber again. However she is unaware about the police are also heavily involved in the war and are working under Orlando Reeve. Assassin fights Orlando's Clan Calatin force. She used several Zabaniya against them until Jester reveals himself and Assassin finally realizes he's still alive. Jester used a Command Spells to teleport her away. Assassin was forced to be sent to the woods.
While in the forest, Assassin stumbles upon False Lancer and Saber, watching them test each other's strength and forming an alliance. After their battle, Lancer reveals that they were aware of Assassin's presence the whole time. Assassin then comes out of hiding and approaches Saber. She recognizes Saber as Richard Lionheart, a King of England who once fought against a Dead Apostle with one of the Hassans. This causes Assassin to form an alliance with Saber and his Master, against Jester. As part of this truce, Assassin was supplied with mana by the mage summoned by Saber's Noble Phantasm in place of Jester's.
Assassin later finds Sigma, who was supposed to summon True Lancer. However, unbeknownst to Assassin, Sigma summoned Watcher, a Servant who cannot be seen by other Servants of interact with them, while Sigma himself is supposed to become True Lancer. Sigma manages to escape Assassin, by distracting her and quickly tries running away. He, however, stops to protect Ayaka, who he believes to be a bystander. This causes Saber and Assassin to decide to form an alliance with him. Assassin, however, remains wary of Sigma.
Assassin and Sigma later talk privately, with Sigma telling Assassin that he doesn't have anything to believe in and put his faith in. This is a large contrast from Assassin, who has extremely strong faith. Assassin though understands that Sigma simply hasn't yet encountered anything to believe in yet, causing her to simply wish that he will eventually find something that is worthy of his belief.
During Clan Calatin's and False Berserker's battle against True Archer over the safety of Tsubaki Kuruoka, a comatose child who is also the Master of False Rider, (with the Clan Calatin wishing to defend her and True Archer wanting to kill her) Assassin arrives on the battlefield along with Saber and Sigma. False Archer also arrives to confront True Archer. Jester is also present, guarding Tsubaki for his own objectives, and is furious at seeing her with someone else and using someone else's mana.
In Type-Moon's April Fools' Day 2015, Assassin is in a female group called Momoiro Hassan Z (ももいろハサンZ, Momoiro Hasan Z?) and is part of the Imperial Roma talent agency, her class is Stealth Idol (ステルスアイドル, Suterusu Aidoru?).
While she was unable to develop an original technique of her own, she managed to learn all eighteen of the past Zabaniya techniques that existed at the time under her Zabaniya: Phantasmal Pedigree. The only exception is Zabaniya: Delusional Illusion, the technique created by the one who succeeded the position instead of her. She paid a great price in blood in order to learn all eighteen ancient techniques, and the pain of dismembering her body and reassembling it was extreme. While she had the ability to accomplish her goals at all cost, she did not teach herself the ability to innovate, the ability to create new techniques. Each technique was a "monumental work" that took a leader an entire lifetime to perfect, but she learned all of them in the span of a few years to the disdain of her compatriots. Most of the techniques she has copied are different from the original ones, some being weaker than the originals or, although Assasin herself would never admit it or accept it, better than the originals.
Zealotry is Assassin's Personal Skill, which grants her great resistance to mental interference and trauma, due to her strong faith.
She has displayed:
- Zabaniya: Delusional Heartbeat
- Zabaniya: Cyber Phantasy
- Zabaniya: Ichor of Reverie
- Zabaniya: Delusional Poison Body (mentioned)
- Zabaniya: Meditative Sensitivity
- Zabaniya: Febrile Inspiration
- Zabaniya: Raving Shadow Flash
 Fate/strange Fake - Volume 2 - Servant status: Assassin
Master: Dead Apostle Jester Karture
True name: - (By the time she gained the properties of a Heroic Spirit she had forsaken her name)
Height/Weight: 163cm 53kg
Alignment: Lawful Good
Noble Phantasm: B+
■ Presence Concealment: A-
■ Zealotry: A
Normally unattainable mental strength can be obtained by having so much religious faith in something that it is beyond the understanding of those around her. Trauma is instantly overcome, strong resistance to mental interference type magecraft is obtained.
■ Zabaniya: Phantasmal Pedigree
Type: Anti-Unit, Anti-Army Noble Phantasm
By allowing the body to freely alter itself, it is an ability that reproduces the past 18 works.
In actuality, harsh body remodeling and the like was conducted. By becoming a Heroic Spirit, the body can freely alter its form.
In comparison to the power of the original works, they are higher or lower on a case by case basis.
- Fate/strange fake Volume 2, Bridging Chapter: "One Day, In the Forest"
- Fate/strange fake Volume 4, Interlude: The Parade of Treachery
- Fate/strange fake Volume 4, Interlude: "The Boy Does Not Believe In God"
- Fate/strange fake Volume 4, Interlude: "Backstage at a Third-Rate Comedy"
- Fate/strange fake Volume 4, Chapter 13: "At Last, a Second, and a Third"