Rikudou Reika

The female Master of Assassin of “Black.” She is the only “Black” Master who is not a member of the Yggdmillennia clan. When she was young, she was a highly cultivated young lady from a wealthy family, but after her parents died in an accident, she fell into a degraded lifestyle in the blink of an eye and, in the present, ended up supporting a certain man while working as a prostitute.
That man is Sagara Hyouma. By using hypnosis to control Reika and make her completely trust him, he used her as a sacrifice to summon Jack the Ripper. However, Jack was summoned before Reika died, and Reika formed a contract with Jack before Hyouma could. The original summoner Sagara Hyouma was killed immediately after, and Reika ended up becoming the Master of Assassin of “Black.”
After hearing about the Greater Holy Grail War from Jack and going to Romania, Reika gathered information and mana while taking advantage of the fact that she's not a magus, which made it hard for the other competitors to find her. She planned to kill the other “Black” Masters, but—
She and Assassin of “Black” met their end in Volume 4.
In truth, she is a unique natural genius and natural monster, the only one of her kind in this work. Her comprehension and observing eye towards everything is abnormal, and she continued to calmly slaughter magi while always maintaining Assassin's superiority in a situation where the two of them had few weapons and little mana. Even before she left Japan, she turned the tables on and killed a squad of Yggdmillennia assassins with Jack's help. In addition, she's extremely proficient with learning languages. She can even smoothly speak in Latin.
She has no hesitation towards murder and possesses no unnecessary emotional attachment to survival, so once she decided to “steal the Holy Grail,” she simply strived undaunted towards that goal. Yet, her wish is nothing special. She simply acts following the optimal strategy, like an AI, for the sake of Jack, who saved her life.
Jack is the only one who she showers with human-like affection. This is because, in her entire two decades of life, Jack was the only one who saved her without asking for compensation. In a sense, Reika is certainly similar to Jack's victims, but she differed from them in a single aspect. Her life was spent being stolen from and trodden upon by others, but she never tried to steal anything from anyone herself. Even though she lived her life going with the flow—spending an empty life that involved just beating her heart, she still tightly grasped onto her own life, at least.
From an outsider's point of view, her life was completely meaningless. A boring, worthless life of only walking aimlessly while zealously hanging onto her heart. But Assassin of “Black,” Jack the Ripper, saw that life as meaningful.
Even if Jack had a cruel innocence that saw her mother's visage in every woman.
Even if Jack would have easily killed Reika if things had happened even a little bit differently.
Jack, who had chosen “not to take her life,” was a dazzlingly beloved child to Reika. That's why, when Jack was targeted by Archer of “Red,” Atalanta, Reika was able to protect Jack with her own body without any regret.
No matter how foolish an act it was.
She ended her life by her own hands without an ounce of regret.

Fate/Apocrypha material: Fate/Apocrypha Encylopedia