What is chakra?
For anyone who has watched the anime, the answer is obvious: An energy resulting from the mix between the physical power stored in the body's cells and the spiritual power of an individual, animal, or human.
Chakra can manifest in many forms and comes in different volumes for each individual.
For some, its extraction and manipulation is a challenge they often fail to overcome. For others, controlling their chakra is as natural as breathing, which gives them a superior advantage over other shinobi.
Clans that do not possess a Kekkei Genkai, a unique manifestation of chakra, rely on secret techniques that have been passed down through their lineage over the generations, as is the case with the famous Ino-Shika-Chō.
Their techniques are not a Kekkei Genkai. They were not born with those unique abilities, but rather trained them.
Other clans that did not develop secret techniques of that style focused on refining training methods to be exceptionally good at an Elemental Release, or a particular combat style, whether utilizing summons or swords.
What about civilians who don't have access to these training methods? Their conception of chakra turns out to be mediocre, and that's why there aren't many outstanding civilian Ninjas.
Sunagakure, for example, did not have any outstanding civilian Ninja in its history, at least none that are known. All their heroes focused on a unique and secret aspect.
Not much is known about the First Kazekage, but he must have been very powerful if he managed to unify the desert shinobi and bring them together in a single Village. The Second increased the Military power by popularizing puppets.
The Third developed the Satetsu, a quite useful and versatile weapon, utilizing the Magnet Release.
The Fourth Kazekage learned that technique, Pakura was born with the Scorch Release Kekkei Genkai, Sasori developed his own puppet-based Kinjutsu, and Gaara, being Shukaku's Jinchuriki, naturally obtained the original Magnet Release from which the Third took inspiration.
For Kirigakure, it was the same. They had clans with many Kekkei Genkai, such as the Kaguya, Yuki, and Karatachi. Others developed unique techniques, like the Hōzuki, and if there was any civilian Ninja who rose to fame, it was due to their affinity with one of the Mist Swords.
In Kumogakure, there were the Storm Release users, who for some reason did not belong to a single bloodline. Perhaps, in the past, there was a clan with this Kekkei Genkai that disseminated among the civilian population, and that's why so many unrelated Kumo-Nin could wield this advanced Element.
In Iwagakure, the Second and Third Tsuchikage mastered the deadly Dust Release, and among their ranks, from time to time, a user of the advanced Steel Release appeared. Additionally, they had secret techniques such as the Explosion Release and the members of the bee clan.
In conclusion, if you don't belong to a clan, or don't receive adequate support, it is extremely difficult for a shinobi to reach great heights.
Not even Namikaze Minato, with his genius, would have been truly outstanding if he hadn't been given the Hiraishin and hadn't married the last Uzumaki who held the remnants of her clan's sealing techniques.
Sure, he probably would have been famous for creating the Rasengan and combining it with a combat style dedicated to pure speed.
But without the Hiraishin, the Yellow Flash wouldn't have existed.
With this in mind, Takeshi found it incredibly difficult to teach Karin much. He did not have access to many techniques, did not master a wide variety of Jutsu, and did not have the support of a village.
Sure, he was a Yuki, but the circumstances that brought him to life resulted in his mother, Kimiko, being self-exiled at the age of only twelve.
She, the woman who taught him almost everything he knew, did not have enough time to grow up within her clan and acquire much knowledge. Because of this, Kimiko barely qualified as a Chūnin in terms of strength, even though her reserves were Jōnin level.
And Takeshi himself? His case is different. His mastery over the Ice Release surpassed that of any clan member, due to the way he came into this world.
That, and his father, the reason for his mother Kimiko's disgrace with the clan, left him some scrolls so he could learn to use his Dark Release.
So almost all the low-level Ninjutsu he mastered was learned from his mother, or acquired from the few opponents who spat Ninjutsu at him in the past, and which he absorbed with his Kekkei Genkai.
What to do then with the redhead?
First and foremost, it was to discover her highest elemental affinity. Based on that, it would be decided whether Karin would become a one-trick pony or manage to be more or less competent as a Kunoichi.
The reason? Kimiko had no idea how to train affinities other than Ice, and Takeshi himself had a mediocre affinity with any element other than Water and his Kekkei Genkai.
A strange thing, since Ice Release is an advanced application of Water and Wind, but Takeshi's affinity with Wind is garbage.
That didn't stop him from using Ninjutsu, but its power and quality were laughable, and the waste of chakra wasn't worth it. It was easier to freeze everything like Todoroki and that's it.
—EEK!— the girl whimpered after a whole hour of boring meditation in the leaf-sticking-to-the-face training. —This is useless, Big Brother Takeshi!
Takeshi simply smiled, amused by the impatience the little girl showed. Wasn't she supposed to be smart and even managed one of Orochimaru's bases?
Perhaps the fear of the snake prevented her from throwing tantrums.
—No, it's not useless. It's the first step to becoming a capable Kunoichi— the young man said, but Karin didn't believe him.
The cart swayed gently over the rough terrain. Kimiko rested on some rags, and Yumi silently knitted some clothes, occasionally looking at her daughter, who was starting to behave like a brat.
—But I want to learn Ninjutsu, not stick this silly leaf to me!— she exclaimed with a pout.
—Hmm— the brown-haired young man hummed before continuing. —Do you know what is required to perform Ninjutsu?"
Karin was about to answer, but realized that, in fact, she had no idea. She narrowed her eyes, her mind racing to find an answer that didn't sound childish.
—Hand signs— she finally said. To her embarrassment, Takeshi laughed.
—That's one part, but... Do you know what hand signs are for?— Takeshi questioned again, but Karin shook her head.
—The signs are a guide, Karin. They help you shape the chakra properly, in the correct pattern, to unleash the Ninjutsu you want. Truly skilled shinobi barely need one or two signs, and some don't even use them. That's because they already perfectly understand the movement their chakra must make to produce the effect.
But neither the signs nor this understanding work if you don't master the basics first: Controlling your chakra.
Control your chakra to stick the leaf. Control your chakra not to fall off trees, control your chakra to walk on water. Control how it moves, where it moves, and when it moves.
Only then can we begin to learn Ninjutsu— Takeshi explained, lighting up Karin's face with every word.
—Whoa! Can you walk on water, Big Brother!?— she shrieked. Clearly, that part was the coolest for her out of the entire explanation.
Takeshi nodded, summoning a water clone that stood horizontally beside the cart without falling, surprising both Uzumaki women. Afterwards, he jumped to the side of the road, where a small lake was brimming with animal life, and the clone ran along the surface until it dissipated after getting too far away.
Karin, seeing the feats she could soon perform, went back to meditating with the leaf for a couple more hours.
For her part, Kimiko smiled discreetly at what her son was attempting. Yes, chakra control was one thing, but that didn't mean Karin had to go through all that to learn Ninjutsu.
She guessed that the goal was to improve her control to teach her that mythical technique she had only heard about until her son replicated it at the age of eight.
It was that day that Kimiko understood one thing: Takeshi was a genius. Her son was a unique genius, and that both excited and terrified her.
Although, of course, she couldn't know that he hadn't replicated that technique on his own. He simply knew the steps to do it, which, in this world, was a mystery that only three people knew.
And unknowingly, Karin would join the seven in the entire shinobi world to use it, although that was debatable, as the future had already changed.
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A/N:
I count Minato, Jiraiya, Naruto, Kakashi, Konohamaru, Hiruko (from the movie), and Boruto. I haven't seen the Boruto fanfic, so I don't know if any other character there also uses it.
Is Hiruko not canon? All of Boruto shouldn't be canon, but it is what it is.
T/N: Did Hiruzen also master it?