Hinata Hyuuga was a very kind girl, despite being born into a very cold family as heiress of all things. During one of her training sessions, her father activated a Caged Bird Seal on her uncle. She was only 3 years old at the time. It left a very powerful impression on her. For many weeks after the incident she would wake up screaming in cold sweat, seeing various members of her family tortured in her dreams.
Her equally kind mother was the one who helped her during those nights. She was also the one to explain how the clan was divided, and what seals were. Even being so young, Hinata understood the value of the seal in protecting her clan. Equally, she never wanted to see anyone else in her clan hurt like her uncle ever again. This led her to swearing to fix her clans seal. Everyone should be wearing it with pride, including main family, and not be afraid of it being used for punishments.
Shortly after the incident with her uncle, a Kumo ambassador tried to kidnap Hinata. It not only further traumatized the kind girl, but also lead to the death of her uncle. Despite trauma, this event only solidified her goals. New seal was the best way to stop foreign shinobi targeting her family.
By the time Hinata was 5 years old she already was able to read, and it was rare to see her without a scroll or book about basic seals. In fact, she even started to draw the most basic of seals. It made her mom very proud of her. Her dad, on one hand, approved her seal obsession, because it was big part of their clan heritage. As heir it was her responsibility in the future to apply the Caged Bird Seal. It was a very complex seal that required solid understanding of seals to be used. On the other hand, Hinata was doing very poorly in their clan's fighting style – Jyuken, which was a very important skill to be proficient in for the clan heir.
The same year her mother gave birth to her younger sister Hanabi, and died shortly after due to birth complications. Death of her mother devastated Hinata, as her mother was the only one in the clan who was kind to her. She was Hinata's rock and support, and now she felt like the ground was taken from under her feats. It took couple of weeks for Hinata to gather her resolve, and she took to studying seals with renewed vigor. After all, she promised to her mother on her death bed that she will not allow Caged Bird Seal being placed on her younger sister's forehead.
Her dad could hardly be called affectionate while her mother was alive, now he become downright cold. Her training sessions became harsher and his displeasure at her Jyuken failures more obvious each day, often ending with her in infirmary. As result, she became even more closed off, and more obsessed with seals. Branch members often had to fetch her for meals, as she would forget and skip them. The only one she was close to was her sister. More often than not she would be studying in her sister's room, sometimes while holding her little sister in her arms.
When she was 7, she already progressed to the experimental seals, which were dangerous to work with directly. The safest route was in using solid clones for testing. This led to Hinata discovering her Suiton affinity as well as realizing that she was very good at the Suiton Jutsu.
Her father was less than happy to discover that she was using her time studying elemental Jutsu while still less than proficient in Jyuken. He would often send branch members to confiscate any scrolls or books not related to what he saw as appropriate materials. Thus, starting Hinata on her journey of developing intricate ways of hiding "contraband" materials from her father and branch members.
As 8 years old she joined ninja academy, which was unusual for a Hyuuga heir. Her father and clan elders already dismissed her as someone they could mold into the "perfect" clan head and instead focused on training her 3 years old sister. Hinata was still very close to her sister, but her father and elders were putting efforts into separating them, to ensure that Hanabi would not follow in her older sister's footsteps.
Being in presence of so many other kids who were not afraid to express themselves, many from other big clans with their own specializations, gave Hinata a lot of new ideas for seals to experiment on. Which was very nice, as she was running out of study materials for seals.
Even though her clan had probably one of the biggest libraries of sealing arts in Konoha, the sealing materials rarely were more advanced that required for application of Caged Bird Seal. However, unlike with buildings, when demolishing something was easier than building, with seals the opposite was happening. It was much easier to apply a seal, than it was to disable one. It often led to explosive results that causes extensive damage to the area around seal, the medium the seal was applied on, and in the case of storage seals to a content that was sealed.
Sealing arts required a steady hand, extraordinary penmanship, sharp mind, as well as lots of time and efforts. All of which were helping Hinata grow the spiritual - Yin part of her chakra, while the Yang part was not developing quite as fast.
This kind of imbalance was quite common among ninjas. For example, both Inuzuka and Akimichi families had excess of Yang chakra. Therefore, their Jutsu tended to use more of it that Yin part, which resulted in physical changes to their bodies. Yamanaka, Nara and Kurama families have excess of Yin chakra, therefore their Jutsu tended to use more of it and had much smaller physical effects on the environment.
Elemental Jutsu tended to use more Yang chakra. Genjutsu was heavily skewed to Yin side. Pure chakra Jutsu required different levels of balance between Yin and Yang chakra depending on the Jutsu.
Wanting to increase her overall chakra reserves and be able to move into stronger seals, Hinata had to increase Yang part of her chakra. This mean additional physical training. It took only couple of weeks in the academy for Hinata to decide on the training regime that suited her. Then she started working on seals to augment it.
Initially she went for simple weight seals for her wrists and ankles, however soon realized that those were not optimal to give her body balanced workout. As some of her muscles had to work extra hard, while others got little to no additional workout.
As a growing seal master, she strived to optimize everything. After all, nearly any shinobi can turn a scroll into exploding scroll with minimum of seal training. It took an artist to condense seals to create exploding tags. Masters could condense seals to a quarter of exploding tags size while keeping the same exploding power.
Obviously, provided some seal training and ready design with instructions, even people with minimal seal training were able produce exploding tags, otherwise price for those would have been astronomical.
Over the following couple of months, she used all of her skills to mix, refine and combine many different seals into single design with a goal of getting the best workout and fastest increase of her Yang chakra. The final seal, her best work to date, consisted of 3 main training seals and many, many, many stabilizing and controlling seals.
After all the sealing array covered her whole body, and it would be unpleasant, to say the least, if the seal was to be damaged and destabilized for any reason. Be it injury, chakra injection via Genjutsu or Jyuken strike, or something as simple as her getting slightly bigger as she grew older.
The main seal of the array was a modified barrier seal, that projected a skintight barrier over her body. Every time she moved, she pushed against the barrier making any sort of movements more difficult. She had a bit of scare with an early version of the barrier. She didn't account that when she was breathing her rib cage was moving.
Those 10 breathless seconds were terrifying when she realized that she couldn't breathe. She was so confident about it too. When she applied the seal on her clone, it had no problem with it. Thankfully, she only pushed enough chakra for the barrier to last 1 minute and with her panicked movements, she was able to drain it in 10 seconds.
The next version of the barrier had provision to restrict slow movements less than fast movements, so breathing was much easier than walking, but walking was much easier that running, and running was easier than punching.
Then she added a form of memory to her barrier, so if she pushed her arm out, then the barrier form became of her with her hand outstretched. This way she had to fight barrier again to retract her hand, rather than barrier pushing it back for her.
Another benefit to her barrier was that it was two ways. While it contained her in, it also kept things out from entering the barrier. Any attack on her was lessened or completely stopped by her barrier. Which was very important since she was slower and her attacks weaker while the barrier was active.
To work on her endurance as well as to increase the workout at times when she was not actively training, she added some weight seals. Which essentially were modified storage seals, when weight of whatever stored inside was not transferred into the pocket dimension but rather added to the sealing medium. The extra weights also added the benefit of her strikes being much stronger while the seal was active, or much faster when it was deactivated.
Knowing that she may need to remove the weights at moment notice Hinata had to modify the seal in a way to be able to easily move weight to the pocket dimension at moment notice. It took a bit of experimenting with the seal, but she was able to get the result she wanted in the end.
The goal of the last seal was to increase her durability and ability to take damage. It worked as if she were walking on surface of another planet with greater gravity than that of Elemental Nations. However, it was very dangerous for her health. Normal weights only caused risk of pulling muscles or breaking bones, unless you do something dumb like dropping your hand with weights onto your face.
Additional gravity, on the other hand, put pressure not only on muscles or bones, but also on organs, making heart pump harder for blood to reach brain, lungs needed to provide more oxygen for harder working muscles, digestion pulling more minerals faster from food, and so on.
Organs were much harder to train than muscles, and too much pressure may cause significant health problems or even death. For those reasons Hinata opted to take it slow, and only increased her gravity by single percent point every two or three weeks, while continuing her normal physical workouts.
The barrier seal consumed a lot of chakra, especially when she was actively training. In addition to fueling the barrier, she often had to use chakra to augment her body speed and strength during exercises. Which mean her chakra coils and network had to work twice as hard, and her chakra pool depleted much quicker. The results of this were quicker charka recovery rate and faster growing chakra pool.
The weight and gravity seals were also increasing her own mass in relation to environment, so she had to apply a bit of chakra to soles of her feet, or she could break some surfaces as she walks on them. While her weight increase was not substantial at this moment, she wondered if she would have to learn to apply some chakra subconsciously while sleeping, as it may cause issues in the future. This constant adjustment of chakra while she walked or run, as well as constant expenditure while she stayed still resulted in her chakra control growing quite rapidly.
The first couple of weeks were very hard on her, but every time she saw her sister Hanabi, cousin Neji, or any other branch members of her family or even her orange wearing classmate, her resolve solidified, and she pushed on.
During this time, she also discovered and corrected numerous small problems with the seal that could not have been foreseen without using it for prolonged periods of time. All her initial tests lasted for less than 10 minutes.
One of the bigger problems was that she ran out of chakra very quickly and when it happened, all three training seals were disabled. This was a bad design as weight seals consumed next to no chakra, so there was no reason to let them be disabled other than by choice. To correct this problem, she added a small buffer of chakra dedicated to weight seals, which ensured that those would not be disabled unless she wished for it to happen.
The gravity seal required continuous inflow of chakra, however the amount of chakra it required was lower than her chakra regeneration capabilities. This mean she should also keep it active at all times. She fixed it by increasing minimum level of remaining chakra for the barrier seal to deactivate from 5% to 10% of her reserves. Thus, the gravity seal will only deactivate if she continues to lose chakra even after the barrier seal deactivated. This way the barrier seal would be disabled first, and it should allow for gravity seal to continue operating.
The 5% was the minimum level any ninja should go to without suffering severe chakra exhaustion that may take days or even weeks to recover from.
The new design led to her barrier seal lasting even less time, so to fix this problem, she added a much bigger chakra seal. It would siphon her chakra into the seal when her reserves reached 95% capacity. This way she could store some chakra while sleeping or on her days off to be used to prolong training.
She made sure that the seal siphoned chakra from all of her Tenketsu points equally. Different Jutsu and chakra skills required stronger chakra flows through different points. Developing all of her points equally allowed her to learn a wide variety of Jutsu while keeping all avenues for specializations later in life open.
Instead of using stored chakra to directly power the barrier, the chakra was used to refill her own chakra reserves if it fallen below 75%. This way she was able to use stored chakra not only for her physical training, but also to train her Byakugan, chakra control exercises, Jutsu and even used it to power seals she didn't have reserves to power on her own.
In parallel to the Yang seal she was also working on other seals she came up with while observing her classmates and older academy students.
When she had her second seal array finished, she wrote both seals on separate scrolls, rolled them up, locked them and wrote a single name on top of each.
Next day in the academy, when everyone went for a lunch break, she stuck into her cousin class and left the scrolls on two of the student desks. Then quickly and quietly left for her own lunch without anyone noticing.