Assassin of Black

One of the Servants of the “Black” camp… or so they should have been, but she deserted and took action independently. Their true name is Jack the Ripper. The serial killer from London who is the most famous of their kind in the entire world.
Usually, serial killers fall into two general categories. The orderly type and the chaotic type; those who can commit murder while maintaining a certain amount of reason and order, and those who commit murder based on reasons that are completely illogical and no one besides themselves can understand. The first type is overwhelmingly harder to catch, since the second type does not even destroy the evidence of their crimes. And Jack the Ripper was the chaotic type.
Even though much evidence was discovered, Jack the Ripper was never caught. It has been cited to have been due to the limits of scientific investigation at the time and due to the initial investigation getting a late start because the victims were only prostitutes.
On the other hand, despite being a chaotic type serial killer, Jack the Ripper also displayed actions such as sending a letter to a newspaper and inciting a panic. Though there is a strong possibility that the letter was a fake, there was no evidence that could completely prove it was a forgery.
The reason that Jack the Ripper is, in a certain sense, so universally extolled throughout the world is due to the overwhelming number of mysteries surrounding this serial killer. Therefore, when Jack the Ripper is summoned as a Servant, he or she can undergo various changes depending on the class or location he or she is summoned in. These changes continue to occur, except in the case where the “true Jack the Ripper” was fixed and settled by the Holy Grail.
…The Jack the Ripper that was summoned this time is an amalgamation of the children who were aborted by the many prostitutes who lived in White Chapel, London. Having not even being allowed to be born, these boys and girls sought to return to their mothers' wombs and repeatedly committed murder. To put it another way, this is the heretical Jack who was born from the “side of the victims.”
However, even they themselves do not know for certain whether or not they were “Jack the Ripper.” After all, they are an amalgamation of evil spirits, so their memories are always vague and, even if they remember having killed prostitutes, it's uncertain who exactly they killed. What they kill as Assassin is not a specific individual, but rather the society that killed them, and the strong emotions they carry cannot be understood by any other hero, making them beyond salvation.
Perhaps due to their great fame, they stand out as an excellent Servant. Due to being a murderer, their efficiency as a “soul eater” is extremely high, and even with Rikudou Reika who was an ordinary person and virtually unable to provide any mana as a Master, they were able to continue maintaining their first-rate battle power by continuing to kill others for mana.
On the other hand, their difficulty in being handled by their Master is the highest among the Servants of this Great Holy Grail War. In the first place, it is dubious whether they even understand the fundamental duty of a Servant to “obey their Master.” To them, the present world isn't somewhere to return to, but rather nothing more than a strange and unknown place… In other words, they hold no lingering affections for the world of the living.
Considering that, the compatibility between Jack and their former master Sagara Hyouma was pretty much the absolute worst possible—or rather, perhaps it is better to say that all magi have the worst compatibility with Jack. Rikudou Reika is the only Master that is compatible with the existence known as Jack.
After being sublimated by Jeanne's hands and having their binding as an amalgamation of evil spirits collapse, they will never be summoned again. Even if the conditions are exactly the same next time, a different “Jack the Ripper” will merely be summoned.

Fate/Apocrypha material: Fate/Apocrypha Encylopedia