Yin chakra, in its most basic form, guided creation.
Used in a human body to govern imagination, it was the force of nature that allowed things to be made.
While Yang chakra was great for giving strength and form to existing things, only Yin could form something entirely new.
Which is why, when Minato split the Kyuubi’s chakra into two halves, he perhaps should have reconsidered which half he sealed away.
When Kushina’s barrier around her birth site finally fell, Hiruzen rushed across scorched earth to find the corpses of the new parents lying next to…
A small child.
Holding a kunai.
Aiming it at a baby.
Hiruzen caught the child’s hand, and watched in horror as tiny, pupil-less red eyes stared up at him through a glare. The child's skin was strangely ashen, and on the top of his head sat the true source of the cold trepidation in Hiruzen’s gut: two tiny, smooth horns, just as could be found on renditions of the Sage of Six Paths.
“Leave me, worm,” the child - no, this was no child, it was a demon - hissed. “This whelp has my chakra. I refuse to be trapped, even in part.”
Sarutobi stretched out his own chakra, and barely kept himself from recoiling at the feeling. Though twisted and split, that was definitely the Kyuubi’s chakra. Half lay in the childlike form before him, and the other, in the crying infant he'd been attempting to kill.
Hiruzen looked to the baby, taking in Minato’s blond hair and Kushina’s light brown skin, and let out a low breath. Minato had sealed the chakra of the nine tailed fox into his son, but not all of it. Somehow, the other half had taken it's own form.
“You are weakened and outnumbered, Kyuubi,” Sarutobi warned. “Surrender, and we will find a way to reunite your chakra when the boy is old enough.”
The Kyuubi growled at him. “I will not let myself be trapped!” He repeated. “I'd die here before I became a prisoner again.”
“I can't let you harm Minato’s child,” Sarutobi said. “You have already killed his parents.”
For some reason, the Kyuubi turned away at that, staring off at the horizon. He got his explanation when the child-demon looked back, face grim and eyes ablaze with fury.
“I did nothing,” the Kyuubi said. “I did not break the seal on my own, and my actions against the village were not my choice. I was controlled. A Sharingan user took command of me.”
Sarutobi blinked. “An Uchiha? Or..?”
“If I had to guess at his identity, I'd name him Madara.”
Sarutobi let out a low breath. “That is...a hefty accusation. A lot to take in faith, and my faith is not something you have much to spare of, Kyuubi. Not when half your chakra lays sealed in a boy I never got the chance to hear the name of.”
“Naruto,” the creature murmured.
“His name,” the Kyuubi spat. “Minato wanted to name him after the character in that perverted old bastard’s book. Kushina called him Naruto in her last words.”
Sarutobi looked to the baby - to Naruto . “Naruto, then. Uzumaki, since we can't let him become a target.”
The Kyuubi laughed. “And his mother's name will save him?” He waved to the infant. “Barely an hour old, and he's already his father's spitting image. No one is going to be fooled as to what happened here.”
“Then I may need to send him away,” Sarutobi said. “He will not be treated fairly, as a jinchuuriki.” He looked to the Kyuubi again. “But first, to decide what to do with you.”
The Kyuubi held out his tiny, pale, human hands, examining them carefully. “I..have a deal. If you will accept it.” He looked up with the deep red eyes that had startled Hiruzen so. “If not, I'll do things my own way. You'll have to take me down fighting.”
“What deal?” Sarutobi asked, because he doubted he'd have very much of a choice.
“Let me stay with my chakra,” the demon said. “It would not be hard to change this form, and pass myself off as a normal child. I could be his cover family - if you will allow me my freedom unhindered. I'll even join your ridiculous ninja army, if you wish. I would rather keep siphoning my chakra back off the kid forever than be trapped in those damned chains again.”
“You want to become Naruto’s guardian?” Sarutobi asked, incredulous. “And you expect me to trust you?”
“Put a fucking watch on me, geezer,” the Kyuubi hissed. “I don't care. I'm not going to kill the kid when my chakra would die with him. Until he's old enough and strong enough for me to extract it and reabsorb it, I'll keep him safe.”
Sarutobi knew it was insane, but…
Naruto would never be safe, as the child of two famous shinobi and the container of the nine tailed fox. This was his best shot, and it gave him the unlikely ally of the Kyuubi himself.
“...You'll need a name,” Sarutobi said, admitting defeat without outright stating it.
The demon-child snorted, and Sarutobi watched him shift, skin turning an even brown as the horns sunk away and his eyes developed tiny pupils in their empty red centers. “I have one. Put it down on whatever papers you have to, but call me by it. I am Kurama, bijuu or human. Don't forget it.”
Sarutobi went to step forward and collect the baby, but Kurama moved first, scooping him up with surprising gentleness.
As though catching his confusion, Kurama bristled, turning to glare at him. “If I'm going to be taking care of this little shit, I need to start now. Just because he stole my chakra doesn't mean I'm gonna let him grow up alone! It's not his fault his ancestors fucked me over!”
Sarutobi blinked, because that was strangely...reasonable. Interesting, coming from a creature everyone claimed was made of pure hate.
Kurama stuck his nose up, holding Naruto close to him, running his sharp nails lightly through short blonde hair until the infant finally stopped crying. “Well? Where are we going, old man? I'm not going to be introduced to your shinobi at a murder scene.”
Sarutobi sighed, and gestured for Kurama to follow. “I will arrange a cover story for you, and a place to stay. And call me Sarutobi. I have the feeling we’re going to see a lot of each other.”
Kurama’s cover was that he was an Uzumaki relative, sent to join Kushina in Konoha when his father (corrected from Sarutobi’s originally 'mother,’ because Kurama would only ever treat one person with the deference one would a parent) passed. He arrived just in time to find Konoha in war and Kushina bleeding out with a baby, and was named Uzumaki clan head and entrusted with Naruto’s care.
Not a bad story. Everyone seemed to accept it easily, when Sarutobi told them, and Kurama received a lot of random sympathetic looks.
Sarutobi, in the meantime, gave Kurama a small apartment and a field promotion to chunnin, citing that he 'fought well’ against the Kyuubi. Kurama had found that particularly funny, but chose to show more of his irritation at being put at a midrank, when he was so far above even S-rank jounin and Kage themselves.
“It draws less attention to have a twelve year old chunin from a field promotion than a child genius ranked jounin,” he'd defended. “Besides, I have no intentions of sending you on jounin missions, if any at all. You'll be given official leave for two years to care for Naruto, and after that, we’ll see about assignments.”
So, Kurama was officially labeled a clan head to a family of two, a legal guardian, and a ninja of Konoha, all within a day.
Not bad, for an ancient fox spirit.
“What the fuck am I doing?” Kurama muttered, letting himself into the apartment he’d been assigned by the Hokage. He looked down to the baby he cradled in his arms, sleeping peacefully, blissfully unaware to the tragedy of his life. As he watched, the boy’s nose twitched, his chubby little face scrunching up as he wiggled it back and forth.
With the whisker marks marring his cheeks, he looked sort of like a fox cub.
“Okay, pup,” Kurama said, straightening himself. “If I want to stop spending all my time trapped in humans’ seals, I have to make this body work. Which means I’m taking care of you from now on.”
The baby sneezed.
“Gross,” Kurama told him. “Sneezing on people is rude, brat.”
Naruto didn’t respond, face smoothing back out as he returned to deep sleep.
Kurama tipped his head back, staring at the ceiling of the apartment as the door swung shut behind him.
Without any further comment, he shifted his form, transforming the body again: painting three long black lines down each cheek, marking him a match for the boy in his arms.
Kushina had been a pain in the ass and a jailor like the rest of them, but she hadn’t been the worst. She was gone, now, because of that asshole Uchiha from the woods, but Kurama remained.
If anyone could teach Naruto how to use his chakra, it would be him. Naruto wouldn’t grow up at odds with himself: both halves of Kurama’s chakra would work together, from the inside out, and shape him into a person worthy of a bijuu.