Just one of those ideas randomly coming up to my mind.
Disclaimer, I do not own Naruto and Zenless Zone Zero!
Hollows, dark and disordered dimensions that appeared out of thin air in the shape of a sphere, swallowing up portions of the world. Anyone or anything trapped inside would either be consumed by Ether corruption or hunted down by Ethereals, mutated plants and animals made of living Ether, born from the Hollows themselves.
No one really knew where these disasters came from. According to history, they'd existed even before the rise of the old capital, which had fallen to the Hollows just a few years ago.
Currently, inside one of these Hollows, a small, rabbit-like white robot sporting an orange scarf and a black jacket with mismatched black and white ears was exploring it. He made his way through the terrain with ease, like he already knew exactly where he was going, until he came to a sudden stop near a collapsed pile of rubble.
"Okay!" A feminine voice rang out of nowhere. "This is the place. Eous, you can start searching now."
The robot, Eous, gave a quick salute in response before turning toward the rubble and digging through the debris, looking for something.
It didn't take long before Eous' small hand popped out from the rubble, holding a small object tightly, followed by his full body.
"Ehn-ne!" (Item found.)
"Great job, Eous!" the feminine voice praised through the speaker, prompting the little robot to shyly rub the back of his head.
"Onii-chan! I got the thing from the commission! Go ahead and ping the client!" The voice continued, calling to her brother as Eous waited, looking around its surroundings.
"Alright, let's head back—" But before she could finish, the robot suddenly turned and started walking off in a different direction. "Eous? What's up? Did you spot a battery or something?" the voice asked through the speaker curiously.
It was rare for Eous to go off-script like this, especially inside a Hollow, since he knew how dangerous these places could be. But if something had caught his attention, then there had to be a reason, so the voice on the other end didn't press further.
The little robot squeezed himself through fallen beams and metal, carefully navigating his way through the wreckage while quietly avoiding the wandering Ethereals nearby.
After reaching the spot where it had seen something, Eous carefully peeked behind a box before his eyes widened. The owner, who was seeing the same thing through Eous' feed, started to panic.
"O-onii-chan! Onii-chan! Get over here, quick!" she shouted, calling for her brother again as Eous rushed over to a battered body lying nearby, a blonde-haired teen in a mesh shirt and orange pants, missing his right hand.
He looked like he'd just come out of a brutal fight. His body was in bad shape, and the stump where his hand used to be was still bleeding.
The little bot panicked, pacing back and forth like a headless chicken. He didn't have any high-end first-aid tools like the newer models, so he did the only thing he could, he yanked off his scarf and pressed it against the wound, using his small hands to try and stop the bleeding while his owners scrambled to come up with a plan.
"What do we do, Onii-chan?! He's still breathing as far as we can tell, but we can't just sit here and wait for Public Security!" the girl's voice rang out, panicked and desperate.
Thankfully, no Ethereals were nearby… for now.
Eous didn't say a word. He just stayed where he was, pressing down on the wound with shaky hands, and hoped that his owners could come up with something soon. They couldn't enter the Hollows for too long so it all depended on him.
"Alright… you wait at the Hollow's entrance, Onii-chan. I'll try to get Eous to drag him out, and you rush him to the hospital!" the girl said, sounding like she'd finally settled on a plan.
"Eous, you heard us, right? I hope you're strong enough to drag him out of the Hollow…"
Eous rolled his small arms, motivated, then grabbed the teen's good hand and started pulling with all his strength.
He budged a little which was a good sign.
The voice on the other end went quiet, swallowing hard.
Eous had to be careful. If he made too much noise, the Ethereals would show up in no time.
Clang!
Right as the thought crossed her mind, the robot's hands slipped, smacking into a steel beam with a loud metallic thud.
"Eous, hide!" the voice urged, and the bot didn't waste a second. He quickly dragged the body with him and ducked behind a stack of boxes.
They stayed low, completely silent, while Eous peeked out carefully from their hiding spot.
Just outside their cover, several two-legged monsters wandered past. Each one had a twisted, unnatural frame and a black orb for a head, wrapped in jagged, crystal-like armor.
"That's... that's way too many..." the voice whispered nervously.
Eous shuddered, staring at the swarm of Ethereals now flooding the area.
They waited for the creatures to move on like they usually did, which unfortunately this time, they didn't.
It was like the Ethereals sensed something nearby, something foreign to them.
Suddenly, one of them let out a deep, guttural roar.
Eous immediately ducked back behind cover and swore if he had a heart, it would've been pounding really fast by now. For a split second, he thought he had made eye contact with that... thing.
Then came another stroke of bad luck, the teen next to Eous let out a low groan and started to stir.
Eous froze.
So did the voice.
"Oh no, no, no, no—"
Eous quickly slapped both hands over the teen's mouth, trying to muffle the sound.
The boy's eyes flew open, only to see something strange hovering over his face.
Normal thing he did at the time? The teen freaked out.
"H-hey! Calm do–" The voice didn't even get to finish before the blonde teen suddenly shot upright, broke free, and leapt out from behind the boxes, straight into the open. The sudden movement was loud enough to alert the nearby Ethereals.
"Wha–?" Still in mid-air, he looked around in confusion, taking in the unfamiliar surroundings. None of it looked like anything he recognized.
"Where the heck am I?!" he shouted, eyes darting from building to building. This place didn't resemble Konoha or anywhere else he'd been. Not to mention he was supposed to be at the Valley of the End, fighting Sasuke… So what was this?
Then something moved in the corner of his vision. A group of twisted, inhuman creatures lunged in fast, all jagged limbs and blade-shaped arms pointed right at him.
"What the heck are these things?!" he yelled, surprised, but his body was already moving on instinct. Whatever these things were, they weren't human, so he didn't need to hold back.
Eous stayed hidden, watching in horror as the teen was suddenly swarmed from all sides.
That's when something happened.
A swirling sphere of yellow energy started forming above the teen's left hand, growing at a rapid pace until it was almost as big as half he was.
Eous watched as he dodged an attack coming from behind, using one of the Ethereals as a springboard to flip through the air. Then, from behind the pack, he slammed the massive energy sphere forward.
The moment it hit the pack, the impact was immediate.
Everything behind Ethereals, the broken-down buildings, the ground, anything caught in the blast, was obliterated like it had been hit by a massive, concentrated weapon.
"W-whoaaaaa!" the voice blurted through Eous' speakers as the dust settled.
In front of them, where buildings had once stood, was now a massive crater, freshly dug and scorched, like something straight out of movies.
Eous didn't move at first, too stunned to react as the silence settled in.
Then, the blonde teen dropped out of the sky, surprising the little robot as Eous took a guarded stance, ready to bolt if he was dangerous. After all, he just leveled an entire block easily.
"H-Hey! Whoa, don't freak out!" the teen said quickly, holding up his good hand. "Sorry for scaring you, alright? I mean, come on, I bet you'd do the same if you woke up with someone trying to smother your face…" He grinned awkwardly at the thing in front of him.
"W-well... you're not wrong..."
"Hey, you can talk!" he said, a bit surprised before crouching in front of Eous. "And wow, you've got a pretty cute voice for a little rabbit!"
"C-cute!?" The voice cracked, flustered, as Eous waved both hands in greeting.
"Yup! You could've given Koyuki a run for her money!" he said with a grin before deciding to move on with the topic.
"Anyway, little guy, if you're awake, that means Sasuke must've finally undone the Infinite Tsukuyomi, right? I mean, no way a summon or animals would be up otherwise… But what were those freaky things earlier? Are they some kind of failed Zetsus?" He scratched the side of his head, still trying to piece it all together.
"W-Wait, slow down!" Eous threw up its tiny arms in protest as the voice snapped out of her flustered state. "What do you mean 'awake'? What even is the Infinite Tsu… whatever? And Zetsus? Are you talking about the Ethereals that attacked you?"
"Ethereals? Is that what you call them?" The teen tilted his head. "And Infinite Tsukuyomi is this illusion tha– wait," he cut himself off mid-sentence and frowned. "Why the heck am I even explaining this to a rabbit? No offense, little guy," he added quickly when he saw the robot fuming at him and stood up. "But I'm really in a hurry and need to go back to my friends and the others…"
Closing his eyes, he reached out for natural energy like he'd done many times before. However, the moment it flowed into him, something felt... off.
It wasn't like what he was used to and the energy here was wild and unfamiliar… almost like it was trying to mess with him or twist him into something else.
He didn't push it away though. Instead, he did what he always did, carefully blended it with his chakra, easing it into balance the way Gamakichi told him.
Too much syrup, and the flavor's ruined. Too much ice cream, and it's just bland. But get the balance just right... and it's delicious.
An absurd comparison, he knew, but for him, Gamakichi's explanation worked well.
It was then he heard something clattering around him. It continued for several seconds until he opened his eyes and looked around.
He noticed the rabbit creature inching away from him as if ready to bolt away in any moments of notice.
"Why are you— wait…" He paused mid-sentence, frowning. "Why does my voice sound weird?" the teen rubbed his throat that felt a bit weird to touch.
Sage Mode never messed with his voice before.
"Y-you can talk!?" The voice squeaked through Eous' speakers, still startled.
"Uh, yeah? Why wouldn't I? I was just talking to you like a minute ago…"
"I-I mean… you look like one of those Ethereals! But, like, more human-ish with scary face!" Eous waved both hands around wildly, as if that somehow helped get the point across. "You won't attack me, right?"
The teen gave the robot a puzzled look, then raised one hand. With a soft poof, an identical copy of him appeared at his side.
"You can clone yourself too?" She geeked out, watching something from movies happening in front of her.
"Whoa! Boss! What the hell happened to you?!" the clone shouted, practically jumping back in surprise.
The original blinked.
His clone wasn't wrong to freak out.
His whole appearance was totally different than usual.
He had some sort of long horns on top of his head resembling their friend's ears, and his face, if you could even still call it that, looked more like a demon's mask with teeth, with glowing reddish-orange eyes burning beneath a wild mess of hair.
There was a soft glow at the center of his chest, pulsing like fire beneath a semi-translucent armor. That same light traced through the cracks and plates of his body, moving slowly under the surface like molten energy trying to break free.
"How am I supposed to know!?" the original snapped back as he looked at his clone, trying to make sense of what he was seeing. The jagged armor covering his body didn't feel like clothing or even metal, it was more like something alive clinging to him.
"Well shit… I can see why you're trying to run away now…" He turned to Eous with an awkward half-smile though the teeth scared the hell out of the robot and his owner. "I look like some kind of final boss or something…"
"See!? You went silent and those… things just started forming all over you!" the voice exclaimed, panicked but clearly also intrigued as Eous cautiously stepped closer.
The little robot tilted his head before reaching out to poke at the glowing section near the blonde's leg. Surprisingly, it didn't feel hot or anything.
"But… wow… This is actually kinda cool…"
The blonde teen rapped his knuckles against one of the plates but paused when he realized he was using something that wasn't supposed to be there.
His right hand... he was using his right hand that looked like a demon's. Sharp claws, armor, protruding horns from the gauntlets and everything.
"…Okay. Yeah. No shit this is cool," he muttered, voice low with disbelief as he flexed the clawed fingers. "Way cooler than my usual Sage Mode…"
The clone, meanwhile, twitched as something prickled down his spine.
He shifted his shoulders and two pairs of wings suddenly burst out, flapping wide with a loud whoosh, blasting all the wreckage including Eous who the original caught at last moment.
"Watch it, you idiot!" he snapped before putting the rabbit-like robot back onto the ground.
"I'm you, so that makes you an idiot too, idiot!" the clone huffed back before he tested the wings, clearly loving every second of it.
"Okay this is starting to freak me out..." The original muttered as he allowed the natural energy slip out of him. The armor covering his body cracked, then flaked away into glowing fragments before disappearing completely. And just like that, his right hand was gone again, leaving behind the same rough stump.
"Holy shit..."
Both he and his clone turned toward Eous, or more accurately, the voice coming through him.
"I just realised… but, did you just absorb Ether energy and then release it like it was nothing? Like you were just breathing?"
"Ether Energy?" the teen and his clone echoed at the same time. "If you mean natural energy, then yeah, I use it for Sage Mode. Though…" He glanced down at himself. "That was waaaaaay different from my usual version."
"Different? How so?" Eous tilted its head to the size, gesturing the voice's confusion.
"Yup," the teenager scratched his whiskered cheek. "My regular Sage Mode doesn't change my whole appearance, just orange pigments around my eyes and my pupils turned toad-like… though lately it turned to cross shapes."
"Did you do it outs–"
"Uuh, boss?" The clone cut in before the girl could finish. "I don't know if you've noticed… but I'm pretty sure this place is some kind of sealed dimension or something."
"... what?" the original whipped his head at the clone.
"Yup," the clone nodded and pointed up. "It's a dome shape. Like, an actual sealed space, about the size of a small city, maybe. But here's the thing... outside of it, I can feel the same natural energy or Ether Energy, whatever, but it's not as heavy, more like what we're used to…"
"Huuuh!?" The original leaned in, visibly confused. Learning there were two different kinds of natural energy was a bit much for him.
"And... the people here..." the clone continued, his tone dropping as he swallowed hard. "They… they don't have chakra…"
His head slowly turned toward the original.
"Boss... I don't think we're in our world anymore..."
"Fuaaaah! That was good!" the teen sighed in satisfaction as he placed down his 29th empty bowl of ramen
Across the table, a pair of siblings sat stunned. The boy with gray hair looked seconds away from puking, his face green at the thought of that much ramen fitting into one person's stomach, while the blue-haired girl just blinked in quiet disbelief.
"Thanks, guys," the blonde grinned, wiping his mouth with the napkin. "I haven't eaten or drank anything for three days ever since before the war started. I promise I'll pay you back!"
"I-it's fine," the brother managed with a stiff smile, clearly still recovering. "Anyway... Naruto, right? From what you said earlier, I guess you're really not from our world, huh?"
"Yup!" Naruto grinned sheepishly. "I think our last clash ripped open the dimension barrier and sent me here... not the first time this happened though. I mean, I got sent into the past this one ti–"
He didn't get to finish since the sister suddenly slammed her hands on the table and leaned in, eyes wide in surprise. "You traveled back in time!?"
"You bet I did!" he said proudly. "I even saved a princess named Sara and met my dad before they sent me back and sealed my memories!" He laughed, leaning back with a nostalgic smile. "I think dying had something to do with breaking the seal..."
"Wooow..." the bluenette's eyes sparkled with wonder as she took it all in.
"Ahem! Belle?" her brother nudged her.
"O-oh, right... anyway," she straightened up, "what do you plan to do now, Naruto? I mean... you must want to go back home, right?"
"Unless you can help me recreate the same attack Sasuke used against me, I probably can't..." Naruto sighed, his mood dipping a bit as he looked off to the side.
"Right... was that the same one with the yellow sphere from before?" Belle asked, her head tilting to the side cutely.
"That's one of its weaker forms, actually." A Rasengan formed in Naruto's left hand. The siblings couldn't help but stare, completely mesmerized by the glowing orb. "The one I used before? That could obliterate mountains easily. And the one I used against Sasuke... if I hadn't held back, it probably would've destroyed the planet."
Both siblings froze in their seats.
"...Wise? I think we have a walking nuclear bomb sitting in our home..." the sister nudged her brother.
"I know..."
"Oh, come on!" Naruto whined as he puffed his cheeks. "I'm not a bad guy, y'know? Even when my best friend was trying to kill me, I held back like crazy to make sure I didn't kill him..."
His exaggerated pout made both siblings crack a smile.
"Well," Wise began, sitting up straighter, "since there's no way we can recreate something that powerful, you can stay here with us for the time being—"
He didn't get to finish. Naruto suddenly lunged forward and grabbed his hand, clutching it tightly with tears pouring down his face in a dramatic display.
"Really!? Wise, my guy, you're a good man!"
Wise leaned back awkwardly in his seat, clearly overwhelmed as he glanced helplessly at Belle, who simply burst into giggles, thoroughly entertained by the whole thing.
"Don't worry, I'll pay for my rent!" Naruto quickly added with a grin.
Belle raised a skeptical eyebrow. "How? I mean... you don't have anything with you right now."
Grinning wider, Naruto reached into his pouch and pulled out a scroll. He casually unrolled it onto the table, then placed his palm flat on it. There was a soft puff of smoke, followed by a thud, as several hefty bags filled with gold coins suddenly appeared, stacked neatly on the surface.
The siblings' eyes went wide.
"Here," Naruto said as he handed one of the bags to Wise, who caught it, only to nearly topple over from the unexpected weight.
"Dude, this thing's heavy..." he wheezed, setting it on the floor with both hands. "How much is in here?"
"A hundred coins," Naruto said proudly. "One of my escorts said they're pure gold. I hope that's enough to cover rent for a while."
Belle quickly untied the bag and peeked inside, her eyes widened even more. Each coin was as big as a baseball.
Without a word, she whipped out her phone and started calculating with an intensity that made Wise scoot away slightly.
"...143 million Dennies per coin..." she muttered under her breath. Then, as if short-circuiting, she blinked slowly. "And you said... there's a hundred in the bag?"
"...A-around a hundred," Wise repeated in disbelief, gently picking up one of the coins and eyeing it carefully.
He turned back to the blonde, jaw slightly open.
"Where in the world did you get this?"
Naruto scratched his cheek with a sheepish chuckle.
"Well... before the war, I was training on this giant turtle island," The siblings' confused look said everything. "Yeah, don't ask... and I kinda stumbled on this hidden room. Whole place was stacked with gold and treasure. I figured I'd give it all to Tsunade-baa-chan once I got back, but... y'know. Now that I'm stuck here, I guess I'll use some of it."
Both Wise and Belle nodded slowly before glancing at each other.
"W-well... actually," Belle said, clearing her throat and clasping her hands together, "we really don't mind you staying with us. In fact, we'd be happy to have you here from now on!"
"That's right!" Wise added with an eager grin. "With this kind of funding, we can finally renovate the whole place and open that video store we've always wanted!"
Naruto gave them both a flat stare.
"I see. So no matter which world I end up in... money still talks, huh?"
The siblings immediately looked away and started whistling.
"Fine... but if you open a store, then... What will the little guy do?" Naruto pointed at Eous who's charging itself in the charging station. "I mean, didn't you guys mention something about commission work or whatever?"
"Eous can help out with the store, of course! And maybe we can even get more Bangboo helpers~!" Belle chimed in excitedly.
"What the hell is a Bangboo? And here I thought the little guy is a rabbit summoning..."
"We'll get to it later," Wise cut in with a wave of his hand. "Anyway, about that commission stuff you overheard, since you're going to be staying here, we wanted to run something by you. Totally up to you though, no pressure."
"Though we're kinda hoping you'd say yes..." Belle added, lowering her voice and fidgeting slightly. Having an otherworldly superpowered ninja as their associate could help them greatly in achieving their goal.
"Don't worry," Naruto said with a grin, "as long as you're not villains or something, which I seriously doubt, I'll help you guys out however I can."
The siblings let out a breath they didn't realize they were holding.
"Phew... thanks, Naruto," Wise said, offering a smile.
"Yay~!" Belle bounced in her seat. "Welcome to the team!"
After settling things between them, the two siblings took the time to bring Naruto up to speed with what had happened in this world so far and what their job entailed, a new proxy called Phaethon.
Typical Isekai trope.
Sage Mode is different since natural energy is Ether Energy in this world. It changed his whole form instead of small make-up, it looks like the combination of Sin Devil Trigger + Baryon Mode, instead of chakra, it's platings covering his whole body.
He will have cybernetic hand, Devil Breakers.
Enjoy!