Theoretically, it's possible.
The supercomputer had left markers on their DNA. That much was certain; it explained the immunity to memory-wiping and XANA possession after being sent to Lyoko.
But to overwrite one's personality, to interweave their genetic code with code he had drawn from XANA itself... The mere thought made him shudder, the hairs on the back of his neck rising.
It was wrong.
"I can't do it on a person," he tells himself, one day. It would be wrong, and with the clone's unexpected personality, it would be bound to land the both of them in hot water.
He restores the update he'd removed, the one that had enabled William to /learn/ and to be himself, neither of which were ever intended to be possibilities.
Then, sitting in the computer room, he brings him back.
The clone looks surprised, faraway eyes wide as he looks around.
Jeremie begins, hurrying over his introduction, "I brought you back. I'm sorry for putting you away like I did. But I had an idea."
William is quiet.
"The supercomputer can't stay on, so you can't stay here in your energy field body. But if you were to possess someone and overwrite their code..."
Jeremie scans William's face, searching for any indication that the clone was understanding what he was saying. The clone's expression is thoughtful, a sharpness to his eyes that he'd only seen when he'd been imitating Odd. Before he'd removed that update.
"I can't put you in a person," he states forwardly, firmly. He won't hide the truth from him, not this time, "But, if you want to keep living on Earth, I could possibly do it to an animal. Would you want that?"
The clone tilts his head, its artificial mind processing Jeremie's statement. "I can stay on Earth with everyone? With my friends?"
"I can try. But you wouldn't be human." It's strange, really, how much his thin patience holds up. Maybe it's the fact that he'd been able to sleep regularly again— or, perhaps, it was the sheer weight of the conversation. "Would you want that?"
The silence of the room makes the soft hum of the computer sound like thunder. After a moment, William nods. "I want to stay on Earth."
He breathes a sigh. He'd almost hoped that William would say no. "I couldn't call you William anymore, if I did it." Jeremie drums his fingers on the edge of the keyboard and looks towards the clone. "What would you like to be named?"
"I'm William."
He exhales slowly. "I know— I mean, if you weren't called William, what would you like people to call you instead?"
Silence. The clone's brow is furrowed in confusion.
It becomes clear to Jeremie that the AI won't understand the question. He changes tactics.
"What's your favorite thing on earth?"
A dreamy smile spreads across the clone's face, his eyes ever vacant. "The rain."
Jeremie blinks in surprise. Then, he remembered how much the clone had marveled during his first storm; he had been so excited that Yumi had had to bring him inside lest he attract unwanted attention. "Rain, then. We'll call you Rain."
Once he returns the clone to stasis, he updates the necessary codes to ensure the AI responds to that name. Then, after a moment's pause, he opens up a new folder and titles it: Project Rain.
And he gets started.
Seasons change. The project had long been finished—it wasn't too difficult, really, since thanks to Aelita and William's experiences with the Scyphozoa (though he wouldn't dare grant those events any sort of gratitude), he understood on a basic level how possession worked. It was a matter of maintaining it, of making it permanent without the power of the supercomputer.
It's ironic, really, that the clone's original is what provides the most useful data on that. He writes up the script that will do it; he runs it through various simulations, scans it meticulously for hours on end in search of any flaws. It's a program that will upload the clone's personality to a /something/, integrate it within that something's own coding to allow the clone complete control. It isn't a suppression of personality like possession typically is; it is replacement.
Once he is finally content with the result, the project sits on the back-burner for three months. Then, one day, he's in Paris, searching for a birthday gift for his father, and while cutting through an alleyway, he finds a box of tiny puppies. Mutts of various markings and shapes, all of which yap excitedly and nearly tip the box over when he draws near.
His mind returns to the project. 'I can't do it to a person.'
If it was Jeremie who helped him, then he could have all the autonomy he would want— and Jeremie could do it under the guise of having a pet. Then, once he grew strong enough, he'd let him strike out on his own. It's a rash decision, not at all thought out, but in a moment of desperation, he picks one up. He checks it's a male—it's bad enough to have to throw Rain into a different species, and he doesn't want to confuse the clone even further by forcing him to come to terms with changing sex—and that it's strong, and after that, he moves on.
Back at Kadic Academy later that day, he sits on the floor in the scanner room of the factory. The puppy happily eats the mystery meat that the cafeteria had served for dinner. Jeremie watches, tries not to think about how it very well could be the puppy's last meal and convinces himself it's to ensure it has the strength for the procedure. Afterwards, it climbs into his lap, tail wagging as it licks at his fingers. He strokes its back until it falls asleep, content and full, and then, he moves it to the scanner.
Upstairs, he stands before the computer screen, finger lingering over the jump button.
He can still go back. Rain would never know; he's locked in stasis, unconscious and unaware. The puppy could go to a shelter and find a happy home—
'I promised him.'
He draws a deep breath, feeling as though he's about to plunge into the Seine during winter.
'I have to try.'
He presses the key.
After the transfer process, an agonizing eternity, Jeremie rushes down to the scanners.
The puppy sits calmly in the center of the scanner, looking around in bewilderment. When it turns to him, its eyes are electric blue, but that's the only real difference he picks up.
"Rain? Can you hear me? Is that you in there?"
A bark, a confused tilt of the head, and then a nod. He grins from ear to ear and scoops him up, unable to hold back an overjoyed laugh. "It worked!"
It had been a brilliant idea.
It had been a horrible idea.
Things had been going well for the past week. He gave Rain as much freedom as he could afford, allowing him to run about outdoors while he went to class and sneaking him into his dorm to sleep. The idea of feeding him dog food doesn't settle easily with him; he brings him cafeteria food. He buys him a sky blue collar, just so he isn't mistaken for a stray and taken off to the pound, but he decides to wait until he's grown to give it to him. After all, for now, he's small enough to hide if need be— and with his update, he's smart enough that what Jeremie tells him to do actually sticks.
He should have known something was off when Rain didn't want to go outside that morning. But he'd shrugged it off and left him to sleep. When he'd returned and Rain refused to eat, he'd begun to worry.
Then, he had woken in the middle of the night to Rain's agonized crying, and when he'd turned the light on, the puppy had been lying on the floor with a bloody muzzle and smears of red covering the gray carpet.
He rushes him to the factory, heart throbbing, desperate pleas for his worst fears to be unfounded echoing again and again in his mind.
He runs the bioscan. His blood runs cold.
Error messages and alert pop-ups. 'Multi-system organ failure,' one proclaims. 'Data files have been corrupted. Current settings are incompatible with life. Restore to previous settings?'
He wasn't a biologist. He couldn't have /known/ about epigenetics, about the carefully orchestrated changes that the code to life underwent to conduct the symphony that was growth and development.
He was only fourteen. He couldn't possibly have known about mankind's clumsy hand, with its inability to even begin to carry forward such a show. Under man's influence, XANA's codes were designed to be static and unchanging, to be perfect for a specific function for an eternity. Life's, though, were dynamic and imperfect, yet perfect for the forever changing question of purpose. There was no chance that the codes of a computer program had the resilience to maintain their integrity with those changes.
It never would have worked. Especially not with the growth and development of a baby animal.
Jeremie tries to swallow, but his mouth is bone-dry. 'Incompatible with life. Incompatible with life.'
He clicks 'Yes.' When the data is restored, Rain's codes pulled from the animal and returned to stasis in the supercomputer, he bolts down to the scanners. The puppy lies motionless at the bottom of the chamber, so small and so frail. Something in him breaks when he touches his still flank and realizes his fur is already cooling. A ragged sob leaves him— the only one he allows— and then, cheeks still wet, he takes the tiny creature from the metallic coffin.
He should have known. He should have known.
He buries him by the roots of a tree near the manhole cover, using a shovel taken from the toolshed (as locks had never stopped him before).
His friends never find out; he knows they'd have never approved. But he spends the next week grieving quietly, and when he moves the files to the trash, he presses 'Delete' without hesitation.