Might Guy knew a taijutsu prodigy when he saw one.
Konoha's resident martial arts master had met many over the years, on both sides of missions. While somewhat prideful of him, he privately considered taijutsu masters to be the most dedicated and persistent of shinobi. Those ninja who focused on the physical side of combat had an eye for subtlety and finesse that even beat out most genjutsu users he'd come across. Perhaps it was simply hubris, but it often seemed as though the miniscule motions of muscle groups required to understand the various strikes and blows, when combined with the speed at which one was expected to shift and move through them, as well as the degree of personalization needed to fit each individual's unique body...
Intellectually, he knew that many of these difficulties had analogues in the other disciplines of shinobi techniques, but in the privacy of his own mind he allowed himself to consider his own the most complicated and under-appreciated.
"Yosh! Kota-kun, I will come at you again now, ready yourself!"
"Don't warn me! Just come at me!"
The point of his musings on his art though, was ultimately just that it was easy to learn, but difficult to master. Much like many different skills, amateurs or hobbyists often considered the words 'learn' and 'master' to be synonymous. He couldn't even begin to count the numbers of genin, chunin, and even jounin he'd tutored on the side in the spirit of YOUTHFUL CAMARADERIE to sharpen their skills who only thought to practice a move some dozens of times and believe they knew the punch, kick, lunge, or throw as well as it could be learned.
"You are truly the fastest opponent my age I have ever fought against, my friend!"
"If you can still talk, you can fight harder!"
"YOSH! You are correct, I will-AH!"
"Pick! One! Fight! Or Talk!"
There were slow, developmental levels to true mastery of any subject. One did not just watch someone perform a technique, then do so themselves for a few minutes, and attain 'mastery' over whatever the technique was. Even if one hypothetically could on an intellectual level, an individual's body would not understand the motions as well as their mind could. That was why many Uchiha tended to injure themselves, sometimes very badly, when they attempted to simply copy his taijutsu. Outside of a few very rare and strange bloodlines, there simply wasn't any substitute for ingrained muscle memory and dedicated practice.
Except...
Guy watched as the young orphan boy, Kotaro, struck Lee with a perfect Strong Fist blow he knew he'd used against the boy just the previous day. The form wasn't just mimicry either, it was an adaptation of the move, performed with the kind of breathtakingly-flawless finesse that took months of very specific practice to get right. Even Lee was only approaching the level of skill he needed.
"Hold!" Guy cried as the two broke apart.
Instantly, both of his students froze, holding their positions as they-well, no-as Lee began to calm his breathing. Kotaro merely stood stock-still, his breathing so even Guy would have more believed him to be taking a morning stroll instead of in the middle of a high-intensity spar. There was, if one looked closely, a very slight sheen of sweat over his skin, but nothing like the beads of perspiration flowing down Lee's face.
Approaching Kotaro, Guy laid hands on the boy's arm and moved his fingers slowly over the various muscle groups, looking closely for signs that he'd torn something or stressed a bone too far in applying that much force.
For the third time since he'd seen the boy adapt one of his or Lee's moves, he found nothing.
"Guy-san, you did say we were free to use whatever techniques we liked, didn't you?" The brown-haired boy asked pointedly. "If you'd rather I go back to only using Shii-Cho I can."
Guy took a deep breath, restraining his manly tears. Kotaro had been quite insistent that such patterns of behavior did not agree with him and, if he still wanted to lure the boy into being his student, Guy knew he would need to make small compromises.
Once young Kotaro agreed to some manner of tutelage, he could acclimate the boy to accept a more YOUTHFUL standard of behavior.
"I was simply making sure you applied the stresses to your body correctly, my youthful young student!" Guy declared with a wide grin, desperately restraining the tears which threatened to burst forth from his eyes. "That move in particular is deceptively difficult once you apply focused chakra to enhance the force behind it!"
Kotaro blinked, cocking his head. "Oh, I didn't use chakra."
Lee perked up, his eyebrows rising at that revelation. "But-!"
Guy held up a hand to his apprentice as he focused his gaze on Kotaro. "What do you mean you didn't use chakra?"
The boy shrugged slightly, a single jerk of his shoulders. "I have a very limited amount of chakra because of the way my system is deformed. I generally... well, instead of using chakra, there's a way that you can... hmm, the best way I can describe it is building up momentum and power by moving a certain way using your entire body as a single unit."
Guy stood from his position kneeling to examine Kota's arm and took his chin in hand as he puzzled out the words. "The entire body as a single unit?"
Kotaro nodded. "Yes sir. When I throw a punch, I throw it in a way that builds force through my entire frame from my feet into my hips, through my torso, and into my arm by flexing specific muscles in sequence."
Guy stared at the boy harder. "That runs the dangerous risk of throwing you out of balance with the requirements of various taijutsu techniques if your muscles are otherwise occupied boosting each other."
Brown hair shifted as he shook his head. "No sir, you just have to be able to do it in the split-second you anticipate the strike to hit, and only a fraction of your body will be occupied at any given point in time if you do it in the proper sequence."
To his credit, Kotaro gave no indication that he was lying throughout the explanation, because the taijutsu master's automatic assumption for anyone else would be that they were lying.
But this young orphan boy...
It would explain a great deal, actually. The way he moved, even among friends and civilians, was akin to how some taijutsu masters he'd seen had also moved. It indicated a hyper-awareness of each and every muscle being shifted at any given moment. What Kotaro was intimating here and now was beyond even that impressive feat. No, to use the body as a 'single unit' implied a level of control over his own body that dwarfed even Guy's own.
"Show me," Guy ordered finally. Turning to his apprentice, he nodded in approval at how the boy was still holding his position. "Lee! Cool down exercises, pattern three!"
"Yosh!" Lee nodded, leaping to obey. "Yes, Guy-sensei! Today I will endeavor to cool down twice as hard!"
"That's the spirit, Lee!" Guy grinned and gave the boy a raised thumb before turning his attention back to young Kotaro, who was looking between himself and his young ward, looking as though he desperately wanted to ask something, but could not bring himself to do so. "Retrieve your sword, Kotaro!"
Waiting until the boy was at his pack, he raised his voice again. "The blade, please. I do appreciate you using the reverse-blade to train Lee how to fight swordsmen, but I wish to see you at your best."
Kotaro paused, turning to watch the jounin with a single eye, before nodding once and selecting the other blade strapped to his training pack. "I suppose you are the boss. You're paying Sagara enough for it." He stopped again, turning back to Guy. "You said my best?"
Guy grinned and nodded, eyebrows rising as Kotaro drew the second blade from its sheath, holding the reverse-blade in what Guy assumed was his off-hand left grip. The boy casually spun the two blades with a confident and casual air as he stepped up opposite the green beast.
"Come at me with intent to kill," Guy instructed, his grin fading as he gave the grave instruction.
Kotaro grimaced, but nodded again. "If you die, I will never forgive you."
Guy's eyes narrowed and he took the signature Strong Fist stance. "Begin when you are ready."
Kotaro closed his eyes, inhaling deeply, before-
Instinct screamed in the back of Guy's mind and he hurled himself upwards.
-Kotaro flashed back into view, his sharpened blade flowing through a smooth diagonal strike which would have taken Guy's head off at the shoulders. Capitalizing on the extension, Guy fell to the ground with the force of gravity behind him as he spun himself into a punch-
Chakra surged into his fist as his weight rocked Kotaro before he bounced off the sharpened edge of the reverse blade held in a reverse grip guarding at the boy's back. It was a testament to how well-made the sword was that it still left the slightest of hair-thin cuts on the jounin's palm even with the reinforcement from his aborted strike at Kotaro's apparently-unguarded rear.
"I thought you did not use chakra?" Guy asked after he landed, clapping his hands together and feeling hot blood rush through his veins as his curiosity began to get the better of him.
Kotaro shook his head. "I did not use chakra in the specific strike I used against Lee. You asked me to do my best, so I am using what little chakra I have in this spar."
Guy chuckled as he caught the barest hint of reprimanding Kotaro's voice, then eased back into his stance with a dark grin. If that was how the boy wanted to play it-
Another body-flicker brought Kotaro within striking distance, this time a half-meter above Guy's own head as he twisted unnaturally to avoid Guy's retaliatory strike while his blade-
Both blades!
Guy threw his body into a spin at the sudden realization that the right-hand sword was coming at him in the boy's other style, the one he was teaching the Uchiha girl, while the off-hand left sword was moving in the style he was teaching Tenten. More than that, though, they were moving independently of each other, as though two different swordsmen were each holding a blade.
Rolling out of the pincer move, his eye caught the collision of blades, the left reverse-blade catching on the guard after sparking down the length of the other sword. Guy's hand caught the ground and his foot lashed out in a downward kick as he balanced on his single right palm.
In response, Kotaro completed his motion and pierced the reverse-blade into the ground as his own foot met Guy's descending shin with a force that could not have been produced by a child not yet out of the academy. It wasn't at Guy's best, true... far from it in fact, but Kotaro still should have dodged rather than-
The barest glint of steel made Guy's unoccupied hand lash out far faster than he'd moved so far, catching the blade held in Kotaro's right hand as it came for his neck again.
A moment passed as the two silently regarded each other, both still upside down; Guy balanced on his hand and Kotaro gripping a sword embedded in the soil of the training ground.
It was perhaps not the most bizarre position he had found himself in during a training match as his spars with his YOUTHFUL RIVAL Rin often saw the medic contorting him into strange positions, but... maybe it would make his top twenty?
"You have still erred in disarming yourself by using your sword to equate our heights in order to make this blow, young Kotaro." Guy stated with disapproval.
Kotaro's face betraying nothing, his other foot lashed out against Guy's free leg, catching the man by surprise as he felt something pull-
The boy's sword rose out of the ground as he felt Kotaro's chakra grip onto his own body, using him like a tree as Guy saw the sword coming and kicked out with both legs to throw the boy across the training ground, breaking the hold he'd had.
Flipping himself upright, Guy caught himself on his feet, looking to where Kotaro had landed against a tree before jumping off it onto the ground and-
Guy felt his leg give out from underneath him, dropping him to a knee as his eyes widened and he confirmed his suspicions. "You can use medical chakra through your feet! It's the only way to reliably use tree-walking on another ninja. Their chakra would fight you otherwise."
"I learned it in case Tenten or Satsuki got hurt while practicing," Young Kotaro stated with a short nod. "It's come in handy a few times since."
Guy considered that, if he could not have Kotaro as his student, perhaps his YOUTHFUL RIVAL could lure the boy into the medical profession instead. Chuckling again, Guy raised a hand coated in medical chakra to the muscles the boy had severed. "Still, you should capitalize on such an opening, Kotaro. It is not often even one so deep in their SPRINGTIME OF YOUTH as yourself catches a jounin off-guard!"
Kotaro raised a single eyebrow. "I applied an explosive seal with my other foot."
Guy paused, eyes widening as he noticed the seal on the thigh of his jumpsuit. The deafening explosion and blinding flash of bright light sent him careening backwards as Guy tried to get his bearings, pumping chakra into his body to reinforce against the coming blows as he reached out and caught each of Kotaro's swords in a tight grip as they came in for another decapitating blow.
Clearing his throat of the remaining smoke, Guy grinned at his student. "I see I have underestimated the FLAMES OF YOUTH which burn brightly within you, young Kotaro! I will hold back less in acknowledgment of your skill!"
Kotaro looked him dead in the eye. "This is the part where I try not to die, isn't it?"
Guy laughed. "I see you and my apprentice have been swapping training stories!"