Uzumaki Miho burst into her room while dragging Shinichi with her.
Her room was a disaster of scrolls, ink bottles, brushes, and papers all covered in calligraphic notes and fuinjutsu diagrams. There wasn't a single piece of furniture besides a futon buried under more scrolls.
"Hehe, welcome to my room! Other than my family, you are the first, boy to ever enter, datteba!"
Shinichi stared at the chaos
"….I don't like older girls. Anyways, let's get down to business. You don't need to teach me directly. Just provide me with your clan's fuinjutsu scrolls. In return I will teach any ten ninjutsu you choose until the war ends."
"Hmph! You lack taste, young boy! And I am no girl! I am a proper lady, Datteba!"
Miho puffed her cheeks indignantly.
"And what does the war have to do with our deal? If you want all my jutsu, you must teach me all of yours, datteba! Equivalent exchange!"
"No. It's not equivalent. You only have fuinjutsu. I have elemental ninjutsu, kenjutsu, shurikenjutsu, and medical ninjutsu. So. Choose ten."
He handed her a scroll listing his techniques.
Miho pouted but still exchanged it with basic and intermediate Uzumaki Fuinjutsu.
"What makes you so sure I'm only good at fuinjutsu?!"
Shinichi opened the scroll and scanned it with his sharigan.
"You're the Three Tails Jinchuriki. And the rumored late youngest daughter of the Uzukage. Clearly, you were heavily spoiled. You have talent to become jinchuriki, but you didn't train properly. And your room is filled with fuinjutsu practice sheets. So yes — you focused on fuinjutsu."
Miho froze.
"H-How did you know that this is my study room, datteba?!!! There is a Futon in here!"
"… Is that really the part that shocked you the most?" Shinichi looked at Miho with raised brows.
Miho collapsed onto the futon dramatically.
"Fine! You win, datteba! Everything about you is shocking anyway. You look younger than me by at least five years, but you're calm, mature, strong, powerful, and knowledgeable. And your sword style—"
She sat up with sparkling eyes.
"I've never seen anything so beautiful, Datteba! Controlling flame petals like that…"
"If you practice hard enough, you can do it too."
"Ha! Hahahahaha! Don't try to fool me with the 'practice makes it perfect'." Miho sat up right while crossing her arms and leaned forward. "I may act dumb and spoiled, but that's just an act. It's the best way to get what I want. I know talent and bloodline matter most. I'm an Uzumaki, Datteba! My greatest strength is fuinjutsu."
"… So it is true then? That Uzumaki is a mutated, Fuinjutsu specialized Kekkei genkai of Senju."
Miho shot up from the futons. Only few Uzumaki were aware of this and she was lucky to hear it from her mother during her youth.
"!!! HOW do you know that, datteba?"
Miho jumped up
"There was a brief mention in Uchiha clan records."
"As expected of Uchiha… You're right. Our ancestor, Uzumaki Kagura — or Senju Kagura — experimented on his own kekkei genkai. He ended up mutating it. His hair turned red and he gained abilities to utilize adamantine sealing chains, healing through blood and chakra transfer, and unique chakra sensing. Those traits passed down to us."
"Interesting… It seems like your ancestor was trying to preserve the Senju's vitality Kekkei genkai and succeeded."
"Huh? What do you mean?"
"Not all Senjus have strong vitality and vast chakra pool. It dilutes over generations. Only the main family retains it. The rest rely on versatile range of knowledge and jutsus that they developed and accumulated over generations. But Uzumaki? Almost all of you have strong vitality and chakra."
Miho's jaw dropped.
"You… you're right…"
"Your ancestor sealed the Senju vitality trait into the Uzumaki bloodline. In a way, Uzumaki are more 'Senju' than most Senju branch families."
Miho stared at him, stunned. This is the most interesting theory and story she ever heard in her life. Her eyes gleamed with excitement and stared at Shinichi in a deferential light.
Shinichi finished scanning the basic scrolls and moved to the intermediate ones.
So, what else do you know? Tell me something interesting, Datteba!"
"…." Shinichi lowered the scroll and looked at her. "I thought you wanted ninjutsu lessons."
Miho her inex fingers together and averted her eyes.
"Well… I did. But now that I think about it, I can't learn your elemental sword styles. They require mastery of nature transformation, right?"
"Of course. That is given—"
"Then I am fine, datteba. You can have the fuinjutsu. I just wanted to talk to you anyways."
"…?"
"Hehe. I know my limits. Right now, I need to focus on fuinjutsu to become a perfect jinchuriki, datteba!"
"Perfect Jinchuriki?"
"Yes! A jinchūriki who can use the tailed beast's chakra freely and even form the tailed beast, dattaba!"
"Become the tailed beast?"
"Exactly, datteba!"
Miho lifted her hand. Red chakra bubbled from her skin, forming a crude tailed‑beast chakra cloak.
"See? This is me 'borrowing' Three‑Tails' chakra."
Shinichi's Sharingan spun rapidly.
"This… it feels like the chakra has its own will and consciousness?!! And It's damaging your body?"
"Well, Yeah, after all, it's not my own chakra."
Miho dissipated the red chakra armor. Her arm was slightly reddened.
"Three Tails rejects me every time."
"... I see…..." Shinichi briefly rubbed his chin and asked cautiously, "Is it possible for an outsider to help you subdue it?"
Miho's narrowed her eyes into crescent shape and stared at Shinichi with a mischievous smile.
"Heh~ why do you want to know, datteba?"
Shinichi just shook his head ignoring her antic and continued scanning the scrolls.
"If you don't want help in the future, forget it."
"Hmph! You are no fun, datteba! You will never get married at this rate!"
"That is none of your concern. I'm done with these scrolls. Next."
"Hmph! Why don't you study them first before asking for more, datteba!"
Shinichi made a hand sign. Two shadow clones appeared.
"You two study these scrolls." He turned and asked once more. "Next scroll."
"…. You're unbelievable, datteba? How are you going to repay this favor?"
She handed him adept, expert, and master level scrolls.
Shinichi accepted them.
"…Why are you giving me all these? Are you really not learning anything from my list?"
"It's fine, datteba. My father said Uzushio and Konoha are family. These scrolls are just guidelines for understanding basic seals, storage seals, explosion tags. The real Uzumaki seals aren't in here. And grandmaster fuinjutsu can't be taught and only mastered through own practices and development... or something like that, datteba."
"I see. But I'm an Uchiha."
"So? You're here to protect us. Aren't you"
Shinichi nodded his head silently and resumed scanning. His clones dispelled.
"! So that's the principle behind fuinjutsu."
Fuinjutsu to certain extent is a very basic utilization of Yin chakra and Yang chakra.
Yin chakra is used to imprint a set of order or the user's will. Yang chakra manifests it. If the user is not proficient with Yin and Yang chakra, then the chakra alone can substitute it, but at huge cost. At a higher level, the user must be capable of sensing and manipulating Yin–Yang directly
He looked at Miho with Sharingan.
"The Yin chakra in your forehead... Is that also fuinjutsu?"
Miho froze.
She hurriedly checked her forehead, then sighed in relief when her protector was still there. She removed it, revealing a light purple diamond.
"Yes. This is the Uzumaki Yinfuin 陰封印 (Yin seal)."
"Interesting… If I master this, I might be able to complete Onmyo Sozofukuki 陰陽創造復歸 (YinYang Creation Restoration)."
Miho tilted her head as she heard Shinichi mutter.
"What's that? Onmyo Sozofukuki? It was not in that list…"
"It is a technique I created. Onmyo Sozofukuki 陰陽創造復歸 (YinYang Creation Restoration)."
He explained briefly. YinYang Creation Restoration is a technique that restores biological states using Yin-Yang chakra requiring: Three-tomoe Sharingan used for biological data analysis and storage, Yin illusion-form creation of biological data, and massive Yang chakra to manifest it.
Miho's eyes sparkled.
"Bakana (impossible), dattebane."
"It is. For now."
Shinichi stood up.
"It is getting late. I will take my leave Mi-"
"Miho!"
"Miho….san,"
"Hmph!"
"Anyways, I will repay this favor."
"You don't have to, dattebane. It was fun talking to you. Much better than old men lecturing me all day."
Shinichi ignored that.
"Good night Miho."
Miho watched him leave and smiled.
"Hehehe. What a dishonest fellow. He acts cold, but his chakra is so warm and passionate."
***
Just for lore explanation (according to my interpretation and imagination)
The basic fuinjutsu guideline provides a very simple practices of creating simple explosion tag seal, flash tag seal, and smoke explosion confinement seal. These types of seals do not require deep understanding in fuinjutsu. These tags, as long as chakra is inputted into these seals, they will activate right away. Those who finished this guideline or mastered the basic fuinjutsu scroll are allowed and capable of copying simple conventional seals but not modify them.
The intermediate fuinjutsu guideline provides a little more advanced version of basic fuinjutsu scroll. The seals in this guideline contains seals related to time and space applications, but there are not enough knowledge to modify or alter the seals. The seals that can be copied after this point are non-living summoning seals of different kinds. (Shuriken, weapons, armor, money, etc…)
The adept fuinjutsu guideline is the true start to studying fuinjutsu. From this point the topic and knowledge are more detailed and complex. This is the bottleneck of whether a ninja can become fuinjutsu specialist. This knowledge provides methods to create and establish basic barrier seals and basic fuin seals with assist of premade seal scroll and material.
The expert Fuinjutsu guideline provides detailed knowledge and materials for user to create more durable and advanced barriers and seals without assist of any form of premade seal scrolls. Also they can form contract seals with summoning beasts and animals and create summoning contract scrolls. This stage requires the user to be capable of sensing yin and yang chakra.
The master fuinjutsu guideline provides knowledge that also modify the conventional seals and any type of seals without accident or messing up. This stage requires the user to be capable of utilizing yin and yang chakra with ease.