Aka Yagyuu Munenori 柳生宗矩. In real life Yagyuu Tajima no Kami Munenori 柳生但馬守宗矩. Sasaki Kojirou is usually the character portrayed asMiyamoto Musashi's 宮本武蔵 greatest rival. This may be because very little is known about him. Historical accounts often mention a duel between a man and Musashi on an island now known as Ganryuu Island. It was originally named "Funashima," but may have been renamed because of the duel. Different accounts give different versions of Kojirou's name, and some call him Sasaki Ganryuu or Ganryuu Kojirou. Musashi himself does not mention Kojirou, and it can be assumed he never really spoke of the duel, though other historical sources have mentioned it.
In the game, a man shows up to defeat Musashi when he is trying to flee from the treachery at the temple after the Sekigahara incident in which he was tricked into killing Tokugawa Ieyasu's 徳川家康 son, Yuuki Hideyasu 結城秀康. This man gives the name Sasaki Kojirou. With his long dachi and Shinkage-ryuu attacks, Musashi finds in Kojirou an opponent he cannot defeat. He decides to live to see another day and flees, but will always wait for a chance to fight this fierce rival again.
Later, Kojirou appears in the Gion district in which Musashi is hiding under the assumed name, Kiryuu Kazumanosuke. More treachery seems at hand, and Kojirou's appearances are brief but seem to revolve around the young girl, Haruka 遥, who hired Kiryuu to get revenge on the man who killed her family.
When Kiryuu faces off against Kojirou at last at Kiyomizu Temple as Kojirou holds a sword against Haruka's neck, Tokugawa Ieyasu intervenes. Kojirou cannot go against his old lord, and withdraws. Ieyasu points Kiryuu to the village of the Yagyuu family, telling him to ask for Yagyuu Sekishuusai 柳生石舟斎, who will explain everything to him.
Indeed, he does. It turns out that Yagyuu Muneyoshi 柳生宗矩, also known as Sekishuusai, patriarch of the Yagyuu clan, is Kojirou's father. Munenori is his real name.
Munenori has forsaken the way of the swordsman, leaving his village with the Shinkage-ryuu skill taught to him by his father. In order to build a han for the clan (a domain recognized by the Tokugawa bakufu--quite a powerful asset), he has decided that he would need to become part of the forces conspiring to change the world. Namely, he decided to join up with Nankoubou Tenkai 南光坊天海, chief advisor to Tokugawa Ieyasu. But, as his father has noticed, Munenori has become a mere puppet in the hands of Tenkai and now struggles, carrying out assassinations about the land.
Kojirou has decided that he needs to become a strong oni, and that he needs to destroy the swordsman identity named Yagyuu Munenori and become the man named Sasaki Kojirou. He intends to do this by fulfilling his last wishes as a swordsman and defeating the legendary Miyamoto Musashi. And so, he tells Musashi of a plot by Tenkai to hold Haruka, a member of the Tokugawa family, so that he hold the bakufu in his hands. It seems she is at the now-famous Ganryuu Island, and that Kojirou has told Musashi this in hopes that he would come for their duel.
In the end, Munenori realises his place as a swordsman when he is defeated by Musashi. He becomes a swordmaster for the Tokugawa forces.
Munenori really was the sword instructor for Tokugawa Hidetada, and again for the third kubou, Tokugawa Iemitsu. He was admired greatly by Katsu Kaishuu, a very important figure in the events of 19th-century Japan. It is known that he fought in the Battle at Sekigahara. His exploits did actually gain him a han, the Yagyuu Han, in what is now Nara Ken."