Kiritsugu Emiya watched as his adoptive children played in the yard.
Haru and Naru.
He found them as the sole survivors of the Fuyuki Fire. Both a blank slate without memories. Haru was older, Naru was younger. In fact, she's barely left her diapers.
But while Haru was near-death that he had to implant Avalon in him to save his life with what little mana it had left from Saber, Naru herself was another story.
She has a beautiful fire on her forehead that stayed active until they were far away from Ground Zero of that hellish inferno he caused. Her eyes were also amber in color and next thing he knew, he felt a strong harmonization of his existence to her… and it was thanks to this that while the curse slowly killed him based on his strength as a magus, he felt no pain at all but developed a strong need to protect her.
His son Haru felt so empty, purposeless and it gnawed at him. Then there's his daughter Naru who was utterly a baby in a child's body, forgetting how to even walk and crawl that made her stay in hospital with therapists and trainers until she could function like a child she should be. Not only that, the poor girl had the misfortune to be connected to the Root that any magus will do ANYTHING to have her. Her Origin was "Sky" that can also be read as "Emptiness", and has no affinities but has the Sorcery Trait of Deathperation Harmonization whatever that was that he can't train her in it as he had no idea what it is to begin with. Her Magic Circuits were too powerful to be ranked; he felt she could do anything she wished and cause miracles left-and-right.
So he gave Haru a purpose. To protect his foster sister who was so pure and innocent because she really lost everything in the fire which was why she was under therapy for a year straight before she could come home to them.
He wished he didn't have to teach them Magecraft but he had no choice. Naru would be on an operating table if discovered! Thus he started Haru's training every after school to be his sister's protector, and Haru accepted his given purpose, no longer feeling empty. He was happy about it actually!
He took to his "baby" sister well, and became a good big brother.
Kiritsugu had another way to teach Naru.
Because unlike him and Haru who had limits, she had none. But of course, he taught her well on what she can, must, can't and should not do with her powers, and the consequences. Easily done because she's a good girl.
Raising her carefully… and modeled her personality after his late wife Irisviel as his "ideal good girl". When her hair grew, he went as far as down to fashion sense.
Because of this, Naru grew up to be a very sweet, endearing, perpetually cheerful… and innocent girl. As both as a human and as a magus. He didn't want her or Haru to be like any typical magus, being heartless souls at a young age.
Being connected to the Root, she has the ability to do anything she wishes, as being connected to it makes her an equivocal existence. She can restructure the laws governing atomic matter, going back in time to change the evolution and development of all life, and reconstructing the orders and laws of the world would be easy for her to accomplish. It is not actually changing the existing world, but instead annihilating the old one with a new reality.
Kiritsugu had to teach her that in a way a child can understand.
However godly her powers are, her limit is her imagination and her own morals as a good girl.
Then Haru is fully-aware that their origin and element determined their fate as magi since day one when he started his education as a magus.
He was a Sword in both Element and Origin, an Incarnation that woulda slapped him a Sealing Designation yet left alone unless he becomes or does something interesting. Being a sword meant his avenues are extremely limited, he can't even do the basic elementary spells all kids learn from their parents before pursuing an art but making him a first-rate specialist in things related to his circumstance. It also explained his desire to have a purpose, as a Sword is meant to be wielded.
His ability in Alchemy lies solely in metal only. He can only learn Material Transmutation, Tracing, Projection and Structural Analysis. He is studying metals he could exploit and forge as weapons. And he is incredibly talented in weapons but hopeless in modern firearms and explosives. He did learn other Magecraft if only from books and learned logic and common sense to defeat Magi who would come after them. He is truly a specialist and inherited all he knew as a Magus Killer, combining modern human development and common sense with magus know-how. And as Haru himself is a sword, he could use magecraft to augment his physical capabilities.
He was also encouraged to think of ways to use magecraft to "make life a sweeter deal"... that led to Haru becoming a House-Husband years later.
Kiritsugu once asked Naru to change Haru's fate as a Magus, but the most she could do was to improve his Magic Circuits because Origin and Element as well as the amount of Circuits is determined from birth upon the soul. She may be connected to the world but only a god can change his fate completely while her own ability is limited. She improved the quality of his Circuits, making it an A from the D it once was, and ensuring his future children will always have A-class Circuits and never degrade, an issue families suffer over time. And from having Avalon in his body, being a Healing and Protective Conceptual item exposed to it for years, she enabled him to learn Healing Magecraft, potion-making and how to make barriers and bounded fields, conjuring said books for him to study in his room. Kiritsugu was thus glad he could change Haru's fate in sorcery to be a better sword for his sister.
As for Illya… he wanted to rescue her so bad. But her disappearing out of the blue the Einzberns would look HERE first and that means exposing the two Sealing Designates in his care. So he had a contingency plan but he would wait when Naru is older for it.
For now, let them have a childhood. He wouldn't last long. Sure, Naru can heal him, but that would mean escaping responsibility.
Those 498 lives killed and two lives stripped of everything they've had and known.
Sacrificing his wife and partner for nothing.
It was all his fault.
This curse and losing his daughter, was the consequence.
But one day… four years later...
He was at his worst.
"Father… why won't you let me heal you?" Naru sniffled as the kids looked at Kiritsugu's bedridden, sorry state, utterly worried and distressed. "All this is over if you'd let me!" she cried.
"Heheh… even if you do, this curse will keep coming back that it's pointless… it's that sort of curse, Naru." said Kiritsugu sadly. "That's why I never asked you. And the Avalon in my body won't work either."
"No way…" Haru choked out.
"I already called a lawyer over as well as old man Raiga… you kids might want to make tea and look presentable. I don't have much time left."
Indeed, he didn't have time left as two days later after making his will wherein the house, land and Kiritsugu's money are all theirs to share between them.
12 years old Haru and 9 years old Naru were upset, looking glumly as his coffin was lowered into the hole dug into the cemetery.
As the first shovel of earth covered the small window that showed his face… It was time for Naru to act.
She erased Kiritsugu's body from existence, along with his Crest that got tainted with the curse.
And they'll be burying an empty casket.
However, while the funeral went on… there was a disguised tiny figure from afar.
He had with him a photo of a little girl, and looked at the crying girl with a crowd at the funeral.
"... I found her. Time to go home soon, piccola principessa."