Disclaimer: Naruto does not in any way belong to me, it's the property of Kishimoto, etc. Likewise, KHR! does not in any way belong to me, it's the property of Amano, etc.

Learning Curve

01.

Kakashi knows he can't win. Even with everything he and Gai throw at Obito - and they are a formidable combination, make no mistake - they can barely even touch their former comrade, much less injure him. Even if Naruto manages to defeat Madara, will he still be strong enough to take care of Obito?

They can't afford to take that chance.

"Gai, cover me," Kakashi says, trying to catch his breath.

"My Eternal Rival-" Gai shoots a worried look at him, as if sensing his intent. Kakashi doesn't think about the fact that Gai knows him better than just about anyone still alive.

"More meaningless words," Obito sneers. "You've realized it by now, haven't you? You cannot defeat me." He launches himself at Kakashi as if to accentuate his point, his movements too fast for the (Obito's) Sharingan to track.

"Now, Gai," Kakashi barks, dodging.

Gai swoops in, forcing Obito to dodge his powerful kick. Obito's giant shuriken still scores a deep cut along his upper arm as the Uchiha moves past him, but Kakashi ignores the pain. Spinning to keep Obito in his sight, Kakashi closes his right eye. In the brief moment that Obito pauses on one of the flying rocks to reorient himself, Kakashi activates Kamui.

Surprise and fury flit across Obito's face before settling into the familiar snarl. "Nice try!" He lunges at Kakashi, free hand outstretched.

Kakashi ignores the drain on his chakra, the protesting of exhausted muscles and the sweat dripping down his brow. His entire attention is focussed on sending Obito to the other dimension.

Obito's hand closes around his shoulder, and Kakashi feels a moment of vertigo before-

-he stumbles onto a paved road.

Obito darts away, a look of confusion on his face.

"This is what Kamui looks like?" Kakashi asks, frowning. He glances around at- the closest thing he can think of to describe their surroundings is a residential area within a large city. Streetlights illuminate the street at regular intervals, revealing uniform fences guarding an equally similar row of mid-sized houses on either side of the street.

"Of course not," Obito snaps. "What did you do?!"

"The same thing you attempted to do!" Kakashi retorts, abruptly remembering that he and Obito are fighting to the death, here. His hand drops to the pouch of weapons strapped to his thigh but-

But the pouch is gone.

A quick glance at himself reveals that he's clad in standard ANBU garb, the wounds sustained during the recent fights completely absent. There's a katana strapped across his back, but he hasn't handled the blade since-

Well, since he was part of ANBU.

He looks at Obito and does a double take. The man - boy? - is dressed in a plain black variation of the Akatsuki uniform, the scars marring his face more pronounced. Fresher. He can't be more than twenty - late teens, by Kakashi's guess. His left eye socket gapes open, empty.

"This is not Kamui," Obito growls, stalking forward. He brings a gloved, half-clenched fist to his face as if to send a katon at Kakashi but-

Nothing happens.

"What happened to us?" Kakashi demands, drawing the katana anyway. It's better than facing Obito empty-handed. He summons lightning chakra to his hand and is only a little surprised when it crackles along the blade.

But it doesn't chirp - it isn't Chidori or Raikiri, just plain lightning chakra.

"Why can you-" Obito breaks off, furious. He makes several other movements, as if attempting other techniques. Except nothing happens, so it's rather comical.

"So you don't know why we seem to have reverted to younger versions of ourselves?" Kakashi enquires mildly, though he doesn't relax his ready stance in the slightest. Obito's still strong, even if he can't use any jutsu.

Now that he focuses, the flow of chakra in Obito's body looks- different. It remains dormant while Obito fruitlessly tries to channel it into attacks, and its form is more reminiscent of flames than the pure energy that Kakashi is accustomed to.

"I-" Obito stiffens, a hand rising to his empty eye socket as if just noticing it for the first time. "Your chakra looks different." His lone Sharingan narrows, and he stalks closer.

Kakashi raises the katana warningly. "Stay back."

Obito's face twists into a snarl. "Activate your Mangekyou," he orders.

"It is-" Kakashi stops, his Sharingan faithfully sharpening the details of his old comrade's face for him. Obito's right eye is red- just red. There are no tomoe spinning around the pupil, and the sharp-edged sickle of his Mangekyou is absent.

He tries to activate it, but it doesn't work. In fact, he can't feel the slow, steady drain of chakra that uncovering his Sharingan usually invokes.

"You can't, can you?" Obito asks flatly, more statement than question. "I can't either. Wherever we are... We're stuck here."

"... Tell me your ideal world isn't civilian suburbia," Kakashi says.

Obito shoots him a murderous glare, made even more horrifying by the gaping eye socket. "Shut up." His eye falls on the crackling katana in Kakashi's hand and he resumes his attempts to channel his chakra.

Kakashi knows that he should be using this opportunity to kill Obito - or at the very least, severely wound him - but he simply... can't bring himself to. Obito is many, many things to Kakashi, but enemy is not one of them.

And then his chakra shifts, gathering in his palm. Kakashi tenses, even as Obito's frustrated expression recedes slightly.

Branches sprout from his uplifted palm, a flurry of leaves rising and shooting towards Kakashi.

It's like no doton that Kakashi has ever seen, but he's not taking any chances - he dodges, alighting on the gate post of a nearby fence. He stumbles, the second nature reflex of summoning chakra to steady his balance failing him. Thankfully he still has his natural reflexes, and regains his footing.

"What the hell is that?" he demands, launching himself along the length of the fence towards Obito. His blade sweeps out, lightning arcing towards his opponent. Green lightning.

Obito dodges, the pavement scorched where Kakashi's lightning struck. More leaves appear around him, somehow threatening despite the fact that they're, well, leaves. The layer of foliage between Obito and Kakashi is so thick that he has trouble spotting the Uchiha through it.

"It's rude to start a fight in another Family's territory," a voice says, behind Obito.

Kakashi tenses, automatically darting to the left; Obito does the same, but in the opposite direction.

A tiny figure stands just beyond the light of the nearest streetlamp. A small child, judging by the size - and the voice was high, young.

"Who are you?" the child adds, with an authority that is completely incongruous when coupled with its (his) small stature.

"Where are we?" Obito demands, his aggressive stance suggesting that he's ready to attack the child.

To be fair, Kakashi's probably isn't much better.

"Namimori, Japan. Now answer the question, please." But it really isn't a request.

The names mean nothing to Kakashi. He can't think of any country or province by that name.

"We're not on the Elemental Continent?" Obito asks.

"'Elemental Continent'?" the child echoes thoughtfully. "No, there's no such thing." He steps into the light, but the brim of his strange hat casts most of his face in shadow. There's a black- thing in each of his small hands, one pointed at each of them. Kakashi assumes they're weapons of some kind, but he's never seen anything like that before. "Names, or I'll shoot."

Kakashi finds himself trading a glance with Obito.

"No?" The child doesn't sound as disappointed as he should. "In that case-"

Kakashi hears the sharp crack before he registers the pain. By then, he's falling from the fence. He nearly bites through his cheek, keeping the shout of pain from escaping when he lands on his injured knee. Damn that attack was fast - his Sharingan couldn't even track it-

Obito grunts as he hits the ground, similarly incapacitated.

The light in the second floor of the house behind the child flicks on, but Kakashi barely notices that.

"Names. Now," the child says, pointing the weapons at their heads this time.

"Hatake Kakashi," he says, glaring up at the child. From this angle, he can see most of the boy's face - no recognition passes across it when he utters his name.

"Uchiha Madara," receives a similar lack of recognition, which is more troubling than the child not knowing Kakashi's name. He's more famous for his moniker of Copy-nin than for his own name; but Uchiha Madara must be a name known to most people on the Elemental Continent. And the child didn't even blink when Obito uttered the name.

"Was that so hard?" the child drawls. "Of course, those names don't tell me anything... So you'll have to answer a few more questions. You still have some intact ball joints left, don't you..?"

"Reborn!" an older male voice calls as someone bursts out of the front door. "Who are you shooting at this time of night?!"

"Intruders, No-Good Tsuna," the child (Reborn?) says coolly. "Now, what's your business here, Hatake and Uchiha? You're no yakuza, but you're not mafia, either."

A boy (Tsuna?) around Kakashi and Obito's current age joins Reborn, frowning down at them. "They're dressed like ninja," he says.

Reborn tilts his head. "Perhaps. Are you ninja? And answer the first question, unless you want a bullet in the shoulder."

"We're ninja," Kakashi says, seeing no reason to hide that fact. Ninja must not commonplace in... Namimori, Japan, though apparently the people here know what they look like.

"That spineless weakling sent me here," Obito adds coldly. It sounds petty, though, and by the way Obito's lips compress, Kakashi assumes that the Uchiha realizes it.

"Ninja," Reborn repeats flatly. Disbelievingly. He turns to Kakashi, though the weapon trained on Obito doesn't waver. "You're the one who brought the two of you here?"

Kakashi grimaces. "We brought each other here."

"Brought how?" Tsuna asks.

Reborn clears his throat, a sound that is somehow menacing, when the silence drags on for several long moments.

"There's a- a technique that we can use to send things - objects or people - to other dimensions," Kakashi says.

"That sounds like-" Tsuna starts.

"-Flames of the Night," Reborn finishes, nodding. As if they've just confirmed a suspicion. "I felt the sudden spike of Flames and decided to observe these two. They really do seem to be from... another world."

Flames? Kakashi frowns. He can only use a few fire jutsu, and Kamui certainly isn't one of those few.

"So you already knew, but you wanted to hear us answer anyway?!" Obito snarls.

Reborn fixes his cool, too-adult gaze on Obito. "Is that a problem?"

"You brat!"

"Ah, I suppose a shoulder isn't that important anyway," Reborn remarks, his weapon moving slightly from Obito's head to the limb in question.

"Stop it," Tsuna says, the atmosphere around the four of them- shifting.

Kakashi's gaze flicks to the previously unassuming young man - a brilliant orange flame is burning on his forehead, his eyes half-lidded and serious as he stares Obito and Reborn down.

Kakashi tries a subtle substitution technique, then a teleportation - sadly (but unsurprisingly), neither works. No chance of escape while the others are occupied, not with his kneecap shot.

"Bring them inside, Reborn, I'll have Ryohei-nii heal them tomorrow. I'm sure we can discuss any problems inside, away from the open street," the boy continues, unquestionably in charge. Kakashi tries not to stare, because before it seemed as if Reborn was the one with the authority here.

"Tch. As the Neo Vongola Primo insists," Reborn says, his expression inscrutable. The weapons disappear somewhere in a feat that would surely make most shinobi envious. "Get up and limp inside, both of you."

"Or what?" Obito sneers.

"Or I'll knock you out and drag you inside myself," Tsuna says calmly.

Kakashi decides to stay on the relatively good side of the strange boys and staggers to his feet. "I don't suppose I'd be permitted to ask a few questions of my own?" he inquires mildly, limping towards the open gate dutifully.

"You may ask whatever you like," Tsuna tells him, fixing that strange gaze on him.

"What are you? That's no child," he adds, nodding towards Reborn. "And you're not an ordinary teenager either."

"Tsuna is the Neo Vongola Primo," Reborn says, repeating the meaningless words to Kakashi again.

"I'm a candidate for the next head of a powerful family," Tsuna answers.

"... Like a clan?" Kakashi presses, trying to make sense of it.

"The candidate. You're the only candidate, No-Good Tsuna," Reborn says.

"Shush," Tsuna says absently, catching Kakashi's arm and steadying him when he nearly falls. "Really, Reborn, did you have to kneecap them?"

"A certain idiot told me not to kill intruders," is the boy's inscrutable reply.

Tsuna doesn't quite roll his eyes, but Kakashi gets the impression he'd like to. "I suppose a clan is accurate, yes," he adds.

Kakashi's about to ask more about Reborn - it seems like Tsuna's influence would be enough to prevent drawing the ire of the boy - but he sees Obito moving out of the corner of his eye and reacts instinctively.

The flurry of leaves that Obito summons is halted when Kakashi manages to create a sheet of lightning arcing from his outstretched fingers to a foot or so beyond Reborn before any of Obito's attack can strike the child.

Reborn already has one of the strange black weapons out again, and Kakashi stumbles as Tsuna simply disappears, leaving only the white after impressions of harsh light behind in Kakashi's vision.

The lightning and the leaves disappear - nullify each other? - just before Reborn shoots the weapon into Obito's other knee, and Tsuna reappears behind, his hands on fire. A swift chop to the back of his neck, and Obito's eye rolls into the back of his head as he slumps forward.

"Don't kill him," Kakashi blurts out before he can stop himself.

Tsuna and Reborn both turn to look at him.

"Why not?" Tsuna asks.

"The two of you are obviously enemies," Reborn adds. "Or... at the very least, he considers you to be an enemy."

Kakashi glances from one to the other, before settling on Tsuna. He's the one in charge here, Kakashi decides. "We were comrades once. I- Many things have happened since then, but he doesn't-" The words catch in his throat; he can't bring himself to finish he doesn't deserve to die. Obito has killed, or caused the death of, many people. Countless people. "I want to fix things, if I can," he finishes lamely, leaning heavily against the fence.

Tsuna studies him in silence for a moment. "You're telling the truth," he says at length, bending slightly to pull Obito over his shoulder. Despite being clad in nothing more than pyjamas - how easy it is to forget, considering the sheer force of presence that the teenager has - and having a rather small frame for his age, Tsuna carries the larger Obito easily. "I don't sense any malice from you, either," he adds, almost as an afterthought, before striding past Kakashi and taking Obito into the yard.

Reborn jumps down from the fence, still gazing at Kakashi will those beady dark eyes. "After you," he says coolly, the weapon still held loosely in one hand.

Kakashi limps after Tsuna, who pauses at the threshold.

"Ah, and Hatake-san?" Tsuna glances over the shoulder not supporting a limp Obito. "If you or your former comrade tries to harm one of my precious people again, I won't hesitate to kill either of you," he says calmly.

Kakashi nods, fighting back an instinctual shiver. "Understood, Neo Vongola Primo-sama."

Behind him, Reborn snorts softly.

"Just Tsuna will be fine, Hatake-san," the teenager adds mildly.

"In that case... My name is Kakashi," he replies, mustering a smile. "Tsuna-san."

He doesn't know anything about this strange dimension, but it seems likely that Reborn and Tsuna are, if not powerful members of society, then well-informed ones at least. Chakra is unreliable at best, and the only person he knows wants him dead. It can't hurt to try and gain the trust of this strange pair.

A/N: I accidentally a Naruto/KHR! crossover. The fic will be Kakashi/Obito later on, though it will not be particularly explicit, nor the focus of the story. Consider yourself warned, anyway!

And why are they de-aged..? Because of reasons- er, I mean, they've only had the Sharingan for eighteen years right so it makes sense for them to be eighteen in KHR!world right. Right. Also Tsuna (& the rest of the guardians) are seventeen-eighteen and Reborn is five (or so, whatever).