The Fourth Holy Grail War begins the first begins but somewhere in a garden in London, a heroic spirit is summoned by John Reginald a magus from the clock tower. The heroic spirit wears a black cloak covering its body, with a crimson and black sheath with a long sword in it whilst it hangs from its side.
[Are you my Master?]
[Yes I am. I am John Reginald your master what is your class?]
[I am Assassin, Master.]
[Well then let’s go to Fuyuki.]
[Master. I suppose you don’t know but we are not part of the current war.]
[What do you mean?]
[I am summoned by the grail to grant your wish not to partake in the war.]
[How is that possible?!]
[A wish by the Holy Grail in order to help someone like you probably. But that is just what I believe.]
He started to cry and then wiped his face with his shirt.
[Can you heal my daughter?]
[Yes. I can.]
[Let’s go to the hospital then.]
They left the garden and went to the hospital John’s daughter was staying at. His daughter was in a coma. Assassin but its hand on her and a light covered her body and her body started to move.
[She should be awake in the morning.]
John couldn’t say a word but noticed that the seal on his hand was disappearing but Assassin was not.
[I can maintain my form without the need for a master’s mana due to my cloak. I guess I will need a place to stay would you mind if I stay with you?]
[Of course! I wouldn’t mind at all. But what is your real name now?]
[My name is Yvain, the Crimson Knight.]
[I thought you were male, and I never heard stories about you wearing a black cloak.]
[That is me from this world’s timeline. I come from a different one. One where I was born female.]
[Wait. You’re a woman. You don’t look like one.]
Yvain takes off her cloak revealing a calm young girl around the age of sixteen with crimson and black hair, clothed in a black and crimson tunic and woolen stockings.
[I am. One of my Noble Phantasm called Black Shroud of the Lion concludes my presence as well as gives me mana to sustain myself.]
[Well then you’re going to need a new name if you’re going to live in London for a while. How about taking my last name and acting as a niece since my family is a mystery to most people.]
Yvain thought it over and showed a small smile.
[Of course Uncle. I guess I should introduce myself. I am Ash Reginald. I will be in your care]
[Pleasure to meet you, Ash. Hope we get to know more about each other.]
The next day Yvain official became Ash Reginald, and entered the Clock Tower as well as entering a British high school. The end of the Fourth Holy Grail War in which Kiritsugu won but the grail destroyed itself causing massive damage, but Kiritsugu’s wish changed the number of deaths and made his wife and daughter turn from homunculus to full humans. Kiritsugu also adopted Shiro due to the fact that Shiro’s family was killed in the initial destruction of the grail.