Disclaimer: I do not own the Fate franchise it belongs to Kinoko Nasu and Type-Moon. I also do not own the Megami Tensei franchise, it belongs to Atlus.

Zero to One

Prologue

The party was in full swing, light shining down from crystal chandeliers hanging high above, falling onto a great gathering of the rich and powerful. Most were on the ground floor, gathered to the sides, some chatting to each other in cliques and small groups, while others watched couples dance on the carpeted middle of the ballroom. Servers went to and fro, taking away empty glasses and handing out fresh ones filled with white wine, or offered finger food in lieu of a drink.

More of the rich and powerful were gathered on the second floor, cliques seated at tables in separate parlors and balconies with cards and alcohol, the better to smooth matters with as issues were discussed and decisions made. Others wandered the halls of the party's host, taking in the sights present, the host having a splendid collection of fine art and historical artifacts on display.

Among that last group was a beautiful woman wearing a sleeveless, off the shoulder dress of red, her dark hair partly done up into a bun at the back of her head, while the rest hung in elegant ringlets down over her shoulders. Gold gleamed around her neck, while diamonds glittered at her ears. Beyond that though, she wore no jewelry, relying on simple, understated elegance to carry the day for her.

She wandered down a corridor, servers passing her and others by, before she stopped before a great, floor-to-ceiling portrait of a man in a 19th Century naval uniform, framed in gilt metal. The woman regarded the painting with collected awe, before leaning down to examine the small placard giving the subject's name and the artist behind such a work.

"Well now," a man's voice said from behind her, and the surprised woman turned to look at a blonde, heterochromatic man in a tuxedo standing behind her. "You must be rather lonely to be wandering so far from the ballroom, or perhaps do you have an eye for paintings?"

"Neither," the woman said with a slight touch of frost to her voice. "I was just sightseeing, that's all."

To her surprise, the man gave a small bow of apology. "I beg your pardon." He said. "It seems I've been too liberal with my words, and started things off on the wrong foot. Perhaps we could start over?"

The woman gave a small smile while raising a hand to her face. "…I suppose." She said.

The man rose, and then gave a courtly bow. "I have the honor of being named Louis Cyphre," he said. "Might I have the honor of having your name, my lady?"

"I have the honor of being named Aoi Tohsaka, Mister Cyphre." Aoi said with a curtsy. "I'm pleased to make your acquaintance."

"As am I, Lady Tohsaka."

"…I've been told this painting here was a gift from someone down on their luck." Louis said, gesturing to the painting Aoi had been examining just a moment ago. "Apparently, they presented it with head bowed, thinking it worthy of our esteemed hosts in exchange for an invitation to tonight's festivities."

"…this might seem rather discourteous," Aoi remarked after a moment. "But wouldn't some say that doing so to be rather…shameless?"

Louis laughed. "I suppose so." He said. "Self-respect is a thing, is it not?"

"Hmm…my husband would say that a gift offered by someone lacking in self-respect devalues the gift offered."

"Oh?" Louis drawled. "And what do you think?"

"…I'd say it's just as discourteous to reject a gift offered in good faith." Aoi said after a moment's thought. "That said, I would extend – in guise – words and advice to encourage the giver to have more respect in themselves in the future."

Louis tilted his head in acknowledgement, before taking a pair of glasses from a nearby server, and offering one to Aoi. Aoi accepted the offer, and Louis raised his glass in a toast. "To your health, Lady Tohsaka." He said with a smile.

"And to yours, Mister Cyphre." Aoi said, returning the toast and smile.

The two drank from their glasses, and Louis nodded in satisfaction while twirling the wineglass' neck between his fingers. "Perhaps you could care to tell me about your husband's work?" he asked.

"Would you tell me about your work in turn?" Aoi asked back.

"But of course," Louis said. "Quid pro quo, yes?"

Aoi laughed and then nodded. For the next hour, she and Louis spoke on their respective families' work. On Aoi's part, she spoke of how the Tohsaka fortune was built on the high – but reasonable – rents levied on land properties in their home city of Fuyuki in Japan, and which her husband Tokiomi had subsequently expanded with calculated investments in various fields both at home and abroad.

Mostly heavy industry, such as steel and metallurgy, shipbuilding and even textile production. Investments were also made in capital enterprises such as banking and insurance, while other investments were also made in the mining and shipping businesses.

For Louis' part, his family's fortune was built largely in the manufacturing field, Morgenstern primarily producing consumer electronics in bulk. They did however have branches in other fields such as real estate, tourism, shipbuilding, and even banking and finance, which had Louis remarking that perhaps he might discuss more serious considerations on that regard with Aoi's husband.

"I'm sure Tokiomi could find you the time for a discussion in detail." Aoi said.

"I'm looking forward to it then." Louis said with a smile. Then both he and Aoi looked away as ushers sounded small bells and declared that dinner was being served. "Shall we, my lady? Perhaps you could introduce me to your husband over the dinner table."

"I'd be honored to, Mister Cyphre."

Aoi sat at a table in a five-star restaurant, a slightly-forlorn air hanging around her. She looked out the window her table was next to, looking out over the glittering cityscape of Fuyuki City, the restaurant being located in the upper floors of a high-end skyscraper.

"Oh?" Louis' voice remarked questioningly. "Am I that late?"

"Louis!" Aoi said, starting at the voice and then smiling, rose to her feet with a bow. "Please, take a seat."

Louis gave his usual courtly bow at Aoi, and then took the seat opposite her. "I apologize if I caused offense," he said. "But given Tokiomi is nowhere to be seen, I was worried that I had fallen behind the agreed time."

"Not at all," Aoi said with a shake of her head, and sitting down once more. "However, an unexpected situation came up, involving matters between our family and one of our old allies, and Tokiomi had to see to that first. It's just that it was so sudden that we thought it discourteous to cancel on such short notice, especially given how mutually beneficial our joint enterprises have been…"

"…that he sent you to keep me company for this evening while he took care of such tedious matters." Louis said with a smile. "Oh very well…I completely understand. And it's not as though the company is an unpleasant one, that much is for certain."

Aoi laughed. "Oh my," she said. "Such a flatterer as ever, I see."

"It's not flattery if it's true." Louis pointed out.

"Tokiomi and Kariya say the same thing." Aoi remarked.

"Kariya…he's your childhood friend, is he not?" Louis asked.

"Yes…my dearest friend…" Aoi said with a nod. It was no surprise Louis could barely remember him, they'd only met once, after all. "The only one I trust as much as my husband."

"Hmm…" Louis hummed with a smile. "That's good…friends…friends are a very good thing to have."

"Yes." Aoi said with a nod. "I completely agree."

The conversation stilled as a waiter arrived to take their orders, and then resumed as the man left with a bow. It continued as the courses were served, and as alcohol was poured, inhibitions were lessened. Soon, one thing led to another, and then…

Kariya Matou walked across the ground floor of the training area, wearing an armor vest over a set of dark-colored fatigues. The synthetic fabric of both were clean, but scuffed and worn from heavy use over the past several months.

A small table was set in a niche in the stone wall, carved – like the rest of the training area – out of the living rock of the mountainside. Kariya picked up his training pistols, and sliding the magazines into place, chambered .50 rounds with solid and heavy clicks. The man narrowed his eyes…

…and then sprang to one side, just avoiding a spray of bullets shooting through the air he'd been standing in an instant before. They shattered against the interior of the niche, smaller than the .50 rounds Kariya had to make do with, but he knew better than to underestimate them.

Springing away in a series of hops, Kariya leveled his pistols, taking a moment to aim before opening fire. No such luck: that moment was enough for Dante to close the distance, Ebony and Ivory already in their holsters, Rebellion swinging in to take his head off.

Kariya ducked down, and rolling back several feet, stopped into a crouch before opening fire. Enough force to break an ordinary man's bones barely fazed Kariya by now, as he let loose with a barrage of fire.

Rebellion whistled as Dante swung it around in front of him at superhuman speeds, sparks flying as he blocked each and every last one of Kariya's shots. Then chambers clicked empty, and Dante smirked as he held Rebellion in a relaxed guard. "Good aim," he complemented his student for the past year. "But not good enough."

"Bufula!" Kariya barked while ejecting his spent magazines.

"Motherf-!" Dante swore as Kariya's ice magic chilled him to the very bone, the sheer cold depositing the water in the air around Dante to encase him in ice. It only took him a couple of moments to break free, but that was enough time for Kariya to come around, while simultaneously reloading his pistols.

Dante smiled despite himself as he avoided another barrage of fire, springing to one side and then up into the air, and again blocked Kariya's barrage of fire. This time, he didn't waste time with banter as Kariya's magazines again ran out, charging in to get close and personal with Rebellion.

Kariya hissed as he dodged swing after swing, each and every one of them only narrowly missing clipping him. Reloading both of his guns under fire was impossible so he reloaded only one, and fired at point-blank range.

"Pierce!" he barked as he pulled the trigger.

Dante caught the bullet with Rebellion's flat side, but the force of impact sent him skidding back across the room regardless. "Bufula!" Kariya barked as a snarling Dante swirled with red demonic energy, and sent it flying towards Kariya with a swing from Rebellion. He barely had the time to cross his arms protectively before him, and then he was flying through the air, to slam against the far wall.

Kariya briefly saw stars before he managed to shake himself out of his stupor, then rolled out of the way as Dante leapt towards him. Sparks flew as Rebellion carved through the wall, and then across the floor as Kariya rolled away and then to his feet.

"Fast on your feet…good!" Dante grunted as Kariya jumped up to avoid a swing, flipping overhead to land on the stairs behind, gun raised in Dante's direction. Dante raised Rebellion to block, knowing from experience Kariya's next move.

"Pierce!" Kariya barked, and Dante was again thrown back, even as his student rushed up the stairs, to the upper levels of the training area. Smiling despite himself, Dante sheathed Rebellion and pulled out Ebony and Ivory, before giving chase.

Sparks flew across the darkened interior of the training ground, even as gunshots echoed in the darkness. Ebony and Ivory spat out .45 rounds, which slammed midair into Kariya's .50 rounds, the bullets breaking each other apart on impact.

Dante pressed the offensive, running across the pillared gallery to throw Kariya's aim off, dashing from one pillar to another. Kariya did likewise, teacher and student trying to keep each other in their sights as they traded fire. Misses flew past with the burning light of tracers, and then chambers clicked empty.

Red flashed across the gallery as Dante charged demonic energy into Ebony and Ivory, and sent it blazing across the gallery. Kariya evaded by jumping, onto and then kicking off a pillar, to land on a landing on a higher level. At the same time, he reloaded one of his pistols, and aiming at a pillar, fired with a word.

"Pierce!"

The pillar shattered from the blast, and sending debris raining down over and around Dante. The demon hunter shielded himself with an arm while dodging the larger pieces, and then jumped up after Kariya in hot pursuit. Kariya zigzagged across another gallery, bullets speeding past, and then jumped up, to another landing.

"Pierce!" he barked again, blowing apart another pillar to bury Dante with.

But Dante had Rebellion out now, and with a swing reduced the crumbling pillar into dust with a blast of demonic energy. It kept on going too, and slamming into Kariya, sent him flying with a cry into the adjoining gallery. Coughing as he forced himself up on all floors, he shook his head while taking aim.

"Pierce!" he spat as he shot at Dante.

Encased in demonic energy, Dante tanked the shot before sweeping out again with Rebellion. Red light flashed and Kariya was sent flying across the gallery, and through a wall onto a stone ledge cut into the mountainside.

Kariya coughed and gasped as he caught his breath, groaning as he felt bruised muscle pressing against battered bone. Then he rolled fast to the side and up to his feet, Rebellion narrowly missing chopping down on him. Kariya sped back in short hops, barely staying ahead of Dante while reloading his guns.

Then he opened fire, but Dante refused to let up, continuing to close while blocking each and every shot. Then Kariya's chambers clicked empty, and feinting to one side, dashed to the other, and then up a staircase towards the mountaintop.

Dante pursued, again blocking bullets using Rebellion, until they finally reached open ground on the mountaintop. Then a lucky shot hit too close to Rebellion's hilt, throwing Dante's grip off, and causing the next shot to force him to drop the sword.

But the fight wasn't over, Dante jumping up and over Kariya while drawing Ebony and Ivory. Landing behind his student, Dante opened fire, Kariya shooting down the first few bullets with his own, only for his guns to click empty.

Dante smirked and fired twice more, blowing the pistols from Kariya's hands. "I yield!" Kariya shouted, a split-second before Dante could put a pair of .45 rounds into his chest.

The moment stretched…

…and then Dante lowered his weapons, and Kariya finally relaxed.

"Looks like in the end I can't really beat you." Kariya grouched.

Dante snorted. "You're ten years too early aiming to beat me." He said. "You shouldn't waste your time trying."

"But you said…" Kariya began.

"…I said when we started that you should focus not on beating me, but on lasting for at least three minutes in a fight against me." Dante interrupted before glancing at his wristwatch. "Let's see…three minutes and sixteen seconds. Not bad…and more to the point, you're never, ever, going to find a magus or executor or whatnot who can fight as well as me. If you can last for more than three minutes against me, well…"

Kariya stared at his teacher for the past year as the half-demon shrugged, and then sighed. "…okay, you said that." He admitted. "But, I just thought…it wouldn't hurt to aim for something higher."

Dante snorted again. "Nothing hurts but your pride, that is." He said.

"Fair enough." Kariya conceded.

"A good lesson to learn then, to know your present limits." A third voice entered the conversation, and causing Dante and Kariya – the former standing next to the latter who sat on the stairs – to turn in its direction. Louis Cyphre walked towards them across the mountaintop, incongruous in his business ensemble, and carrying a suitcase with him.

"Only once you realize what your limits are," Louis continued. "Can you really begin to work to overcome them."

"…not bad, old man." Dante said after a moment. "Though that sounds like something you'd find written on a slip of paper in a fortune cookie from your typical Chinese restaurant."

Louis laughed. "I suppose so." He admitted as he came to a halt before the two men, Kariya having gotten to his feet. "But wisdom is wisdom, and always appreciated in one way or another, regardless of the source."

"Whatever you say, old man, whatever you say."

Louis hummed. "And?" he prompted. "Is he ready?"

"Like I said," Dante began. "He's never going to find any magus or executor or whatnot – at least in this cluster – who can last as long as he can against me. Heroic Spirits will always be dicey, though. Especially when they use those noble whatchamacallits of theirs…but so long as they don't…I think he can handle Assassin or Caster."

Dante paused, and then cuffed Kariya on the back of his head. The latter gaped at the former, who looked sternly at him. "Listen, little man," Dante said while grabbing Kariya by the collar. "I didn't kick your ass into shape for the past year just so some dime-for-a-dozen, cocaine-snorting, Middle Eastern killer-for-hire can put you down like a nobody. If I found out you went out like that, I am going to drag your sorry ass out of the afterlife, before sending it back there myself. Do you hear me, Kariya Matou?"

"…crystal." Kariya said with a gulp.

"Great!" Dante cheerfully said while letting Kariya go.

Louis smiled at the banter, before bringing up the case he'd brought with him. "If you are ready to return," he began while opening the case. "Then here are a pair of gifts to help you in the battles to come: Zero and One."

Kariya raised an eyebrow at the naming, and then his eyes went as wide as possible, before he lifted out a futuristic-looking pistol of some kind from the case. He flipped the safety and the pistol came to life with an electric hum, blue light flickering along the pistol's spine. The Hindu-Arabic numeral for one was carved into the pistol's right side.

Then he looked back into the case, and noted the Hindu-Arabic number for zero carved into the pistol's – no, not a pistol, Zero was much too big to be a pistol, but what to call it – side.

Huh…Zero and One…makes sense in context…I think…

"You sure you want to trust him with this job, old man?" Dante asked once Kariya had left.

Louis just smiled, amused. "Trying to poach this job and extra cash with it?" he asked.

"…maybe…" Dante vaguely replied, and Louis laughed. "…he wasn't a bad student, but Kariya's…he's got issues. And from what I heard from him, he's got personal motivations for taking this job on."

"He's a driven man." Louis agreed. "But even those kinds of men have their uses."

"…trying to see if that holds true for Kariya?" Dante asked after a moment.

"Perhaps…" Louis answered just as vaguely and Dante snorted.

"Right…" he said. "Hey, how come I don't get a gift?"

Louis gave him an unimpressed glance. "Your pistols can adopt the Almighty property at your convenience." He said. "And don't get me started on that sword of yours. You don't need me to give you some fancy new weapons. Besides, I pay you generously enough, don't I?"

"…okay, fair enough." Dante admitted. "In all seriousness though, what's your goal here? I mean, it's not like you really need or even want for a wish machine. You can just snap your fingers or whatnot, and shit just gets real."

"Hmm…let's just say there's something in the Grail that might be useful to me." Louis answered after a moment.

"…really?" Dante skeptically asked after another moment.

"Yes." Louis said before giving a cold smile. "Though, if I'm honest, there's also something personal in this for me too."

"Oh?"

Louis smiled. "Let's just say that me and Kariya Matou have some shared interests between us." He said.

"And you're letting him do all the work." Dante observed.

Louis spread his hands. "It's like a play, my friend." He said in a matter-of-fact way. "Each of us must play our part in the proper turn, or it ruins the whole presentation. And my role won't be until the very end."

"Right…I'll take your word for it. Oh, and speaking of which, when do I collect my back pay for this job?"

Louis smiled, and gestured for Dante to follow.

"This won't happen again."

"Of course it won't."

"I mean it. I…I'm a married woman and you…you're my husband's…you're one of his business associates…"

"…in a different life…maybe…"

"…maybe…"

"…"

"…"

"…"

"…goodbye."

A/N

And here we go, spinning off from my previous FSN x SMT crossover, inspired by conversation with Cubia. Here's to you, honored contributor.

The name references not just Kariya's weapons – courtesy of Louis Cyphre – but also a statement from Fate/Zero's writers. That is, no matter how many times you multiply it, zero will always be zero. Too bad Chaos doesn't give a shit about logic, though. Zero multiplied by Chaos can come out as still zero…but it can also come out as one. And one is enough, when all is said and done.

Thank you, statistics.