The Yoi clan belongs to the Yukigakure no Sato. They are known to be merciless and all members are viewed as both researcher and test subject. Their hijusu involves extensive surgery to improve the body of the clanmember, starting only a few days after they have been born.
The Yoi clan is a clan that is built more around conviction than family ties and blood.
In its essence, it is a small scientific shinobi community that is driven to 'better mankind', so to speak, through medical and genetic research. The ultimate goal is to overcome all human limitations, and removing all weaknesses and imperfections of a body, in order to ascend to the next level of evolution. Every member of the clan is equally a scientist and a test subject here.
Notable personal qualities attributed to most members of the Yoi clan are perfectionism, and willingness to sacrifice one's health and even life in the name of a greater goal, which in this case, is betterment of humans. The common belief here is that the ends justify the means, and that victors are never judged. The research and experiments that the members conduct on themselves and others are a well-kept secret. Still, the clan has a somewhat more open-door policy when compared to other clans, and is willing to accept outsiders into their ranks, if they find ways of proving their dedication to the common goal.
The Yoi Clan first appeared several generations ago, as Land of Snow's effort to create their own clan with a kekkei genkai. The land's brightest minds in both medicine and science were gathered in one place to work on the project, in order to strengthen the Land's military might. Without an abundance of clans, like other countries, the leaders felt their nation ... vulnerable. They were given everything they might require, in order for the research to become successful.
The project worked tirelessly for about a decade, before some results, and realizations were made. The first and foremost realization had been that kekkei genkai, as people called it, was very hard to emulate, or create. Almost all of them seemed like mutations or evolutions, that happened accidentally, with minimum directed effort from the people themselves. It would take a lot of time, trial and error to forge one of these, but results were needed much quicker. So the clan turned to other things. Among them being experimentation and betterment of human frame.
First experiments the scientists conducted on themselves, to ensure secrecy, and to ensure that the 'test subjects' would not turn on their creators. These experiments were very careful, and they did indeed yield results. However, some died in the process, due to mistakes and unforeseen consequences. But it was a price to pay for progress. The scientists were willing and eager to continue their research.
During the time working on the project, the men and women became bonding one with another, and, sooner or later, starting their own families. But this did not make them stir away from their main goal - working for the sake of their country and village. One could say they became fanatical in their endeavors. The newborn children of theirs they used for test subjects as well, in more attempts and inventions. Some died, and it was a great loss for the clan. Great, but ... acceptable.
At this point the Yoi were more of one big family than a mere project. And although their initial goal, of creating their own Kekkei Genkai, was still unreachable, they reorganized themselves as a 'clan'. And continued their work. The second generation - the children, have grown up indoctrinated with the clan's values. They were different from their parents - they were already better, stronger, more resilient. And eventually, they replaced their parents and continued their research.
The practice of using their own children as test subjects was not forsaken, but continued. The second generation was bolder with their experiments, and, subsequently, more successful, having the experience of previous generations to rely on. The third generation and higher were constantly modified, improved, starting from birth, and never ending, until they perish... or ascend to something higher.
Now the fourth generation of 'altered' walk the earth. Nobody but the higher-ups of the Yukigakure know about this clan's practices. Some view it pointless, some - valuable. But all agree that the lengths and sacrifices made for progress are disproportionately high.
Not that the Yoi Clan cares. For them, it is 'worth it'.
The clansmen are divided into three branches: The Military branch, the Research branch, and the Control Branch.
The Research Branch
The Research Branch of the clan are those members who have dedicated themselves to research, inventing new ways of bettering a body, and the like. Many of them are talented medical ninja. However, often due to age and physiological conditions, they cannot conduct as much research on themselves as they would require, so they often conduct it on others, mostly members of the military branch.
The Military Branch
The Military Branch is comprised, mostly, of children of members of the Research Branch. Since birth, their bodies are modified heavily due to experiments, and alterations - they are the guinea pigs sacrificed in the name of progress. Those that manage to survive, become very strong ninja, and serve as the clan's fighting force. If they wish, they could become part of the Research Branch, but more often than not they choose to remain in the martial section of the clan.
The Control Branch
The Control Branch are elected representatives of both the Military and the Research Branch. They are the most experienced specialists in their fields. There are usually 2 representatives per branch, and together they make decisions that shape the clan and its path. They are also the ones to choose the Clan Leader.
The Clan Leader is a person elected by the Control Branch to serve as figurehead and the highest authority within the clan. In order to be elected, one has to earn votes of all 4 members of the Control Branch. Once that is done, the Leader holds the absolute power over the clan.
The clan's hijutsu focus on body alterations and the Yoi clansmen gain abilities with age, as they undergo more and more experimentation on. The improvements are being done gradually, to ensure survivability - too many at wrong age, when the organism is at its most vulnerable, can cause unwanted side-effects, and death. They start small, the alteration becoming more massive as the body becomes stronger.
The Yoi clan's policy towards babies is merciless - the experimentation must begin only a couple of days after they leave the womb. The child's joints and muscles are being injected with small doses of resilin - a protein found in joints of bugs, which enables them of jumping really high or perform lightning-fast strikes. By the end of the treatment course the subject is able of jumping twice as high, and striking trice as quickly as a normal person would, without using chakra.
This extensive period of time is dedicated to reorganizing of the subject's circulatory system, and muscle tissue of both arms and legs. The former has main large arteries moved deeper into the flesh, and having them split into smaller ones, in order to minimize the damage in case of a cut - the subject would not bleed to death as easily. The arteries and veins in general are moved closer to the insides of arms and legs - where they're not as vulnerable.
The muscles also undergo rearrangement - instead of being packed into clusters they become more spread across the surface of a limb. The result, in the end, is ensuring the user not losing control of a limb after the muscle having been cut - several cuts must be made to sever all the muscles capable of controlling the limb's movements.
The third procedure being made is the severance of most pain receptors within one's body. The nerves are singed with directed electrical shock. It makes further surgeries much easier to perform, with limited need of pain killers, and render the subject greatly resistant to pain, and thus, most torture techniques.
At this age more potent improvements are being made. The subject is being introduced to what lies in the future, by having the bones in their fingers replaced with plastic alloy laced with synthetic fibers - material that is very expensive and hard to produce, but is known to be stronger than steel, and does not corrode. The subject's hands are riddled with stitches after the operation is complete. Additionally, the nails are being replaced with sharp ceramic knives, which can extend to about 10 centimeters in length. It is also the time when the subject has to take medicine that sedates the immune system enough to ignore the foreign objects within the body.
At the same time, psychological conditioning training begins. The subject is forced to undergo series of exercises to gain mental fortitude - work under stress, become less susceptible to exterior influence and resistance to hypnosis. The subject is put under mental tortures, in order to teach to resist such better.
The final part of the subject's evolution. If it manages to survive this part, it becomes a valuable asset and a proud member of the Yoi clan. Preparations are being made, and the change is gradual. The first step is the re-arrangement of organs within the body. Liver and spleen are being moved inside the ribcage, the organs there more compacted, however, without any loss in functionality. After that, over the course of time, the bones within the subjects' body are being replaced with ones made of plastics laced with synthetic fibers. The ribcage is replaced with a spheric chamber that has no holes in it, however has a few switches that allow opening it, for surgical purposes. On the outside, the subject would look muchly the same, however.
The plastic bones of forearms have a storage compartments within each, hiding a 20 cm - long knife to be shot out through the palm, and used as a weapon afterwards. The final part to replace is the skull - the sturdiest part in this man-made skeleton. After the operation is complete, the subject's body is riddled with stitches and scars, most being at the back of the head, the spine, and the stomach area.
The subject is now deemed 'whole'.
The subject must take daily medical supplies and frequent examination to ensure the frame remains in perfect working condition, and the body is healthy to perform.