The fourth son of the "Oki"'s family who hosts one of the famous swordsmanship dojos in Kyoto.
"Koshiro" is a twin eggy twin with "Kosaburo" of the third son of the "Oki"'s family.
He was five years old away from first brother "Kotaro", and two years old away from second brother "Kojiro".
And since he was born in a pedigree of samurai, so he became naturally learn swordsmanship with his brothers.
In the "Oki"'s dojo, it imposed more hard training for those who were unable to do.
Until "Kosaburo" and "Koshiro" joined the training, that hard training was imposed on "Kojiro".
However, when "Kosaburo" and "Koshiro" join the training, that hard training will be imposed on "Koshiro".
Because "Kosaburo" was excellent from childhood.
On the other hand at the surface, "Koshiro" wasn't able to demonstrate any talent in training of swordsmanship.
As a result, "Koshiro" will be accused of being 《Shame of "Oki"'s family》 from the surroundings.
And when "Koshiro" reaches the age of sixteen, he will be kicked out from "Oki"'s house and live alone in neighborhood.
Because "Oki"s family couldn't live with 《shame of "Oki"'s family》.
However, mother was taking care of "Koshiro", so he wasn't self-reliant.
And at the same time as "Koshiro" is kicked out from the "Oki"'s house, he starts to show a dual personality.
When the main personality slept in the night, another personality has woken up, he went to the city of Kyoto and was killing people.
These two personality don't know the existence of another personality, and there is no memory of another personality.
And the main personality couldn't kill people and couldn't even attack people.
However, another personality was very brutal, the only purpose was to kill people.
Also, he couldn't give out the result of hard training at the time of main personality, but when he was giving out the result of a hard training and hidden talent at the time of another personality.
"Koshiro" seems to have gained observation ability beyond common sense while received hard training from father and "Kotaro".
And with that observation ability, he was staggering the place to be attacked during training.
Although he was able to avoid attacks, training would be more severe if avoided.
In order not to be that situation, he was staggering the place to be attacked and gained techniques to minimize damage.
And it seems that such things made "Koshiro" unbeatable strength in actual battle.
"Koshiro" can avoid all enemy attacks.
Even if the ability to attack the opponent is immature, unless you receive an attack from the opponent, a good opportunity will arrive later.
And attacking on that opportunity, even immature attacks will work.
In such a way, "Koshiro" has been killing people until now.
By the way, when "Koshiro" killed "Kojiro", he isn't targeted at "Kojiro".
Since the target was indiscriminate, it was a coincidence that "Kojiro" was sacrificed.
"Koshiro" was broken personality due to hatred, but he wasn't a murderer for hatred.
So "Koshiro" waited for someone to come somewhere in Kyoto, and he was indiscriminately killing those who came.
And actually "Koshiro" also killed his father "Gentaro".
But his main personality don't remember also that.