Sword of Faith.

In a world unscarred by the grail war a blade is forged, the fires are no less hot for their lack of magic. A boy staggers through the streets of his burning city, the fragments of his past falling away amid the radioactive ash. Yet in the smoke a smiling face appears, shining near as bright as that second sun. And for a moment, a boy in hell wonders if there just might be a god after all: before the pain whisks him away to dream of swords.

Emiya Kiritsugu had been involved in the project since its inception: dreaming of a weapon to end the war, to end all wars. He never wondered why his "companion" was so eager to play the servant... He should have known that even if there was a road to that distant utopia it wouldn't be her that lit the way. The deceiver earned her name, but her lies paled in comparison to the ones he told himself. Yet for all the years that he had walked that left hand path Kiritsugu's will was yet his own. From a bunker in the distance he saw the fruits of his good intentions, and could not look away.

Something snapped within his clockwork mind and the last thing his comrades hear is the clink of silver hitting the floor before their body's join it. Were he a better man he'd have given then a chance to share in his redemption, but he was never a Swordsman: their path was never his own. And so he ran into the hellfire with a pistol in hand and a prayer on his lips, begging for a chance to prevent anyone else from paying the price of his hubris. To prove that for all he had stared into the abyss he did not dwell there, or else slay the monster that lurked behind familiar eyes.

For most, all he can give is mercy: and for every soul he drags out of the fires hundreds more die in agony. Standing at the hypo-center of the blast, despair bringing him to his knees where flame could not: he sees a toddler stumbling amid the city of the dead, kissed by fire yet alive. And that glimmer of hope is enough to spark a dream of redemption. He adopts the boy and calls him Shirou: they both know that bonds of blood are unnecessary for a family forged of steel. His old estate sees more smiles as a makeshift hospital than it ever did as a home, and if anyone notices how he winces at their gratitude, they are polite enough not to mention it.

He isn't surprised to see a familiar foe after the smoke clears, he has fought enough knights to know that if the need is great enough one shall appear. It is she that hears his confession, she who retrieves the coins from where he in his guilt had dared not tread. He's not the first to set aside the silver. And for all that they are in the redemption business the knights aren't stupid: they know that the Lion likes to make an example of those who leave his flock. And so a polite fiction is established, where "friend of the family" comes to mean some strange cross between witness protection and parole officer. There's too much bad blood between them, too much resentment and lingering mistrust for the word "friend" to ever ring true.

Of all the titles he has accumulated over decades of strife "dad" is the one he is proudest of, it reminds him of the family he left behind all those years ago. He tells his son stories of heroes, of the bravest men and women he ever met: and if Shirou never learns that his beloved father played the villain in his tales? Well… another lie could hardly stain his soul any blacker.

Shirou never shows any signs of magic, but he takes it in stride: who wants to be a hedge mage when you could be a hero? He begs his father to train him, and while he was surprised how little convincing it took: he honestly didn't expect his first "mission" to involve volunteering at a soup kitchen. Once the lesson sinks in the real training starts, Kiritsugu knows that while most of humanities ills are best fought with kindness, some must be battled with cold steel and hot lead.

The cancer comes on slowly, a subtler assassin than it's victim, and at first the symptoms were so easy to explain away. Kiritsugu was always a rather pale individual so if he was paler then usual he just hadn't gotten enough sun. If he bruised easily and was tired and was tired and sore well that was just a consequence of sparring with his overenthusiastic son. He was hardly the only one with such symptoms: it was just a virus spreading among the hibakusha until it suddenly wasn't. By the time they realize it's leukemia Kiritsugu isn't long for the world.

Shirou swears to be a hero as his father breathes his last but heroism is a nebulous thing, in real life the monsters don't stand around in the open waiting to be slain. Under the light of day things that once seemed so certain can be excused as the fantasies of a childish mind. But never forgets his fathers stories, and tries to match his heroes in spirit if not in deed. So what if they call him "fake janitor"? He knew he was a Good Man. Perhaps he would have been nothing more then that, but Lion's memory is long: the Denarians would Return to Fuyuki, and the fate of the world would rest upon the point of a sword…

(Author note: I don't know how common of a name Shirou/Shiro is in Japan, but I mean seriously: two of fictions most epic swordsmen sharing a name like that cannot be a coincidence, especially since both the dresden and nasuverse have the whole alternate reality thing going on in the background.)