In the beginning there was only Light, Darkness and Chaos. Then, after an unknown amount of time, there was creation. And from creation, the Trinity of Realities were formed.
Inferno, the realm of Darkness. Home of the Infernals. Paradiso, the realm of Light. Home of the Angels. And finally, the Human World and Purgatorio, the realms of Chaos. Home of humanity and the supernatural respectively. Of course, the Trinity is only one set of realities in an infinite multiverse.
There exist people capable of drawing power from the various planes of existence, achieving mysticism. These individuals are mainly divided into three groups. The Lumen Sages, wielders of Light typically drawing power from the realm of Paradiso, the Umbra Witches, wielders of Darkness typically drawing power from Inferno, and finally the Twilight Mages, wielders of Chaos drawing power from Purgatorio. Exceptions to this has happened before but are exceedingly rare.
One of these rare, if extremely unlucky (or maybe should it be lucky), exceptions was the young Umbran apprentice was the one simply known as Shirou, who lost her teacher in a massive unnatural fire that would later become known as the Fuyuki-Fire.
The reason she was an exception along with her late teacher to most Umbra Witches was that she didn't strike a deal with an Infernal, selling her soul to the demon in exchange for mystical knowledge and power. She struck a deal with a Heroic Spirit, specifically Morgan Le Fay. Willingly accepting to carry a reincarnated Morgan and grant the witch of legends a new chance in life once she deemed herself ready… and thankfully for her, the who she struck a deal with was a patient soul, willing to wait for her reincarnation for more than a few years. After all, time was rather irrelevant to Morgan since she was a Heroic Spirit.
Shirou's contract with Morgan caused a few changes to not only her appearance but also her physiology. The most notable ones being her left eye turning from its natural amber into an emerald green color, gaining sharp fairy-like ears, retractable wings, that were rather bat-like in shape despite belonging to a fairy (A/N: Shirou's wings are based upon the wings of the Imps of Alfheim Online from Sword Art Online), and last but not least, gaining two extended sections of blonde hair on the back of her head, which caused a relatively minor but definitely noticeable change to her Umbran Garb. Replacing the red shoulder-cape-like element of it with what could probably be best described as something similar to a very long scarf.
Clutching her teacher's broken Umbran Watch tightly didn't dull the severe pain of grief that Shirou felt in the slightest as her eyes, bloodshot from crying, wandered across the blazing ruins surrounding her. She was well aware of the dangers that came with being an Umbra Witch, but this fire wasn't caused by either the Infernals or the Angels. The Infernals, she could understand. She was not just a normal Umbra Witch, but an Umbran Princess, who struck a deal with something (or rather someone) that wasn't an Infernal, selling her soul and condemning her soul to be dragged down to Inferno when she died. The flames just felt… wrong… evil… for want of a better way to put it. She knew what hellfire felt like, due to her teacher and herself having to fend off Infernals that could spew hellfire on the rare occasion that an Infernal capable of doing so showed up, and these flames weren't hellfire.
Through eyes blinded by the remnants of tears, the Umbran Apprentice eventually managed to find something other than the flames surrounding her. An adult man, though the remnants of the tears made it hard for her to see clearly, making the man mostly nothing more than a blurry silhouette, screaming into the night in desperation to find life. Not exactly willing to die yet, Shirou responded to the man and made her presence amongst the burning ruins known, by groaning and coughing as loudly as possible, since her throat was rather dry after crying so much. Shortly after she started making noises, the man she would later know as Kiritsugu Emiya ran over to her and caught her in his arms, as she promptly collapsed in exhaustion, still clutching her late teacher's broken Umbran Watch tightly in her hand.
After taking the girl to a nearby hospital, Kiritsugu Emiya took the opportunity to analyze the few belongings that he found in the girl's possession. Two strange but beautifully crafted watches, albeit with one of them being broken. The one that wasn't broken was originally found hanging around the young girl's neck via a black ribbon. The watch itself was made from a steel-and-titanium-alloy coated with a thin layer of gold. It had several small gemstones throughout it, most of which were emeralds and rubies, with a diamond, roughly the same size as the rather large ones that usually decorated very expensive rings, at the center. The watch also turned out to double as a rather large locket, with a picture of a baby in the arms of a woman that held a very strong resemblance to the girl it belonged to, the baby obviously being the locket's owner and the woman holding the baby was probably her mother. Sentimental value aside, the watch was practically worth a small fortune! But since the obvious owner of the watch was still alive and definitely not willing to part with her property.
The Magus Killer got interrupted from his train of thought as he noticed the girl stirring in the hospital bed. As she opened her eyes, Kiritsugu took note that she had heterochromia, separate eye colors. The left eye being emerald green and the right eye being amber, making her something of a very extreme rarity, since those eye colors and heterochromia were extremely rare in Japan. And she had all three of them at the same time. After noticing the oddly colored eyes, the Magus Killer started to take note on the girl's other oddities, dual-colored hair and oddly shaped ears, sharp and rather long as opposed to round as most people had, leading him to believe that the girl wasn't entirely human. Rather than continuing his analysis, Kiritsugu chose to address the young girl who had by now regained enough consciousness to be aware of his presence.
"I see that you're finally awake, how are you feeling?" Exclaimed Kiritsugu uncomfortably, knowing that he was probably responsible for her grief.
"Tired. Where's my watch and who are you?" Asked the girl groggily, probably a mix of grief and painkillers. The fact that she asked for her watch first rather than for the name of the person who was sitting next to her hospital bed made Kiritsugu think that there was more to the watch than it being a mere trinket.
"Your watch is right here. And as for who I am. My name is Kiritsugu Emiya, what's yours?" Exclaimed the Magus Killer as he handed the girl her watch, which she immediately proceeded to hang around her neck while looking at him with a gaze of suspicion.
"Shirou. That's the only name I've ever had and needed. Though why that is the case isn't something I know." Exclaimed the girl now known as Shirou, musing a bit over why she didn't have a last name in Enochian, the language spoken by both the Angels of Paradiso and the Infernals of Inferno, though to Kiritsugu it sounded like a little more than nonsense. It was clearly a language, but he didn't know a single word of what she just said.
"Anyway, can I assume that you don't have a guardian anymore, or am I wrong ?"
"No. My guardian died in that fire saving me." At that, Shirou's tone turned somber, clearly missing her guardian, who was also her teacher in the mystic arts. "The other watch was hers."
"I see. I'm sorry for your loss. Listen, I intend to settle down here in Fuyuki…"
"Okay? Can you skip to the point please?"
"Right, we've both lost someone we cared about in the fire and… well… Ah! Fuck it. What I'm trying to ask is whether or not you're willing to let me adopt you, making you my adoptive daughter." After a few minutes of thought, Shirou gave Kiritsugu her answer.
"Sure."