Born the son of Kaguya Odori Takeshi, Keisuke grew up in a time that was bathed in blood. People were being killed, turned into slaves, taken for experiments, and worse as normal daily occurances During this time, most of the Odori family of the Kaguya Clan was killed, but Sai was hand-picked at birth to suffer a different type of fate. He was to be their sacrifice in creating the perfect monster. The leaders of the Kaguya Clan paid an Okumo scientist to experiment on the Kaguya toddler, expecting to simply use him as a guinea pig that would die perfecting a process to create a perfect son for the Kaguya clan. However, their hope was instead met with the determination of the boy to live and a successful experiment. Keisuke's growth hormones and regenerative abilities were amplified, causing his body to grow at a much more rapid pace than normal, as well as recover quickly from injuries.
Because of the chemical changes in his body, Keisuke quickly outgrew his peers both in height and muscle. His heightened regenerative abilities allowed him to work his entire body to its maximum capability almost every day without causing permanent muscle damage. As such, Shikotsumyaku awakened in him at a very young age. His astounding growth effects both positive and negative. He never fit in with children his age, but he was a good student and definitely stood out among his peers, though sometimes one would wonder if he was really considered one of them. After all, he was a freak of nature, even for their kind, an abomination to the rest of the world, naught but a bred killing machine. Of course, his rapid physical growth made him rather popular among Kaguya girls and even some women. There's no telling how many secrets he's kept from his teenage days during his free time, which was very little. Most of his time was taken up with training that most would consider down-right abusive. He was pushed to his limits every day, forced to endure as much Taijutsu and Ninjutsu training as his body could possibly endure, his elemental affinity being Water. Other than kunai, shuriken, and senbon, the weapons he was made to train with were always the heaviest weapons he was able to lift off the ground. When his body finally couldn't take any more for the day, he was forced to study the human anatomy and learn every possible way he could break, torture, and destroy it.
Much like any Odori, Keisuke grew up hating what Mitsuo had done to them, but he was fairly quiet on that as he was most things. While he wasn't exactly socially retarded, he just didn't care much for talking. When people saw him looking at them, even in normal conversation, he was often described as seeming to be picking out ways he could kill any person his eyes passed in that exact moment. Having a conversation with him was said to be one of the most intense feelings one could ever have. Some kids would even dare each other to try to have an entire conversation with him as a dare. His stares were so intense that it was hard for anyone to say no to him, no matter what he said. Of course, women are somehow always drawn to men like that, so he did have some 'fans', which of course leads back to the point about there being no telling how many secrets he's kept from his teenage days. However, it also help make him one of the most frightening sights to behold on a battlefield. He would tear through countless people without so much as any sign of sympathy, as if they were just meaningless objects put in his path to be cut down.
When the Hidden Villages formed, Keisuke was chosen to be one of the Seven Swordsmen. The sword he was given was called Kubikiribocho, the Executioner's Blade. It held the look of an enormous butcher knife and had both a circle and a semi-circle made into the blade for the purpose of trapping and decapitating an opponent. It had been created with the ability to regenerate itself using the iron in the blood of its victims, a trait which Sai took joy in, slaughtering enemies that he could have spared simply to bring the blade back to perfection.
However, that is but one side of the man, not that the other side is nice at all either. In his own village, Keisuke is known as a man of few words, but one of the most feared as himself and as one of the Seven Swordsmen. He is not known to be exactly friendly, nor as one of the great conversationalists of Kirigakure. Moreover, he is known to keep to himself, even amongst the few remaining of the Odori family, though he is not quite so intense around them or with those adopted into the family, at least not all the time.
A year and a half after becoming a Swordsman, Keisuke was chosen to go on a long-term assignment. There were certain countries other than the Five Great Nations attempting to build their military forces, and some were thought to be a threat to Kirigakure if they managed to get enough power. Sai was sent to slaughter their armies. He would spend the next two and a half years eradicating military camps and rebel forces. Names that may have gone down in history one day were sent into the eternal anonymity of early death by the strike of his bones or Kubikiribocho. Families were left without their loved ones because of the mere fact they had stepped into his path. There was no sympathy, no holding back in his strikes. Keisuke was a man, a monster without conscience, and he had been sent on a mission to do what he did best, kill. Of course, he did make some time for himself in all this, actually enjoying being able to see things outside the Land of Water. If one looks hard enough, they would likely find a child every so often in the path of his journeys that looks a bit, or maybe a lot, like he did when he was their age.
On his way home now from his years of traveling and slaughtering, Keisuke rides a merchant vessel on its way back to his home, back to Kirigakure. He has heard they've been at war in his absence. Maybe there will be some blood left for him to spill when he gets back… Of course, he'll have to visit 'dear old dad' when he gets there too. Maybe he'll find the old codger on the battlefield somewhere.
