The daughter of the respected samurai Shiba Tonomo, the tomboyish Atsuko was granted the right to train with her brother by her loving father. Despite her unusual privileges, she was still expected to attract the perfect match and be nothing more than a good spouse.
While she was herself skilled, her brother Shiba Ibuka was a one-in-a-generation swordsman. One day, however, as they were tasked by their father to bring back a cow for a feast, the siblings were ambushed by bandits. They fled after Atsuko killed three of them on her own, but Ibuka was paralyzed by fear even as the bandits were about to rape his beloved sister. Furious, Atsuko nevertheless chose to remain silent, to spare her father from the shame of having a coward son and to protect her brother's brilliant future.
Joining the Tokugawa forces
At the beginning of the Aizu campaign in the Boshin War, Atsuko disguised herself and enlisted as a male soldier in the Tokugawa forces to watch over her brother and fight. Atsuko was unmasked by Nakano Takeko, an onna-musha and Japanese Assassin who was tasked with protecting the legendary sword known as the Musashi Masamune from the Templars. Unknowingly, the girl was then slowly groomed in the ways of the Assassins by Takeko who took her under her wing. Together, they eventually thwarted a Templar assault led by William Lloyd and his band of rōnin, who attempted to steal the sword. Atsuko killed one rōnin, until she was confronted to "Masajiro". Luckily gaining the upper hand after the rōnin fell to the ground, she choose to kill the one battling Takeko instead of finishing Masajiro, who fled. Once again, Ibuka acted cowardly, but unintentionally distracted the Templar long enough so his sister could recover the blade he had stolen. Luckily for Ibuka, no one alive except Atsuko and Takeko saw his cowardice, and he was branded a hero for safeguarding the Masamune.
After the attack and the death of its moronic owner, the Masamune was taken by the powerful Matsudaira Katamori, no longer needed to be protected. The Shiba siblings were then assigned to poison the wells of Fushimi as Takeko was informed that the Imperial army would halt there. Posing as peddlers, the Shibas infiltrated the town but Atsuko refused to accomplish her mission after having sympathized with several denizens of Fushimi. Back in the Tokugawa camp, she later discovered that Takeko suspected she was not yet ready to harm innocents in the name of the greater good, hence why she blackmailed Ibuka to accompany his sister and also gave him poison. After she refused to do it and while she was sleeping, Ibuka accomplished the mission. Enraged by their betrayal, she left to join back her ashigaru battalion, vowing to not see or help the both of them ever again.
Still posing as a man, Atsuko took part in the Battle of Toba–Fushimi, killing several enemies. Despite her anger, she once again had to protect her brother's reputation after she witnessed him fleeing the battlefield to hide in a tree. Indeed, she tried to kill Uesugi, the former favorite warrior of Matsudaira Katamori before Ibuka joined his guard, after the samurai swore to tell everyone that Ibuka was a coward. More skilled than her, Uesugi was about to kill Atsuko when a cannonball crushed him to death. Later, after discovering that their father had died during the battle, the link between the siblings was broken when Ibuka pretended to have fought bravely.
Battle of Aizu
Six months later, Atsuko now fully trained as a shinobi, was tasked to infiltrate the camp of the Imperial army besieging the remnants of the Tokugawa forces at Aizu. While she succeeded to recover the attack plan of the enemy, she had to flee in a hurry after being unmasked by a sentinel. However, she was caught by Masajiro, who in order to repay her for sparing his life during their last encounter, let her go.
During the Battle of Aizu, Takeko was fatally shot, and after she begged Atsuko to do it, the latter cut her head to avoid it falling into enemy hands. Atsuko eventually faced Lloyd but was defeated by the famous swordsman. She was saved by Ibuka, who eventually overcame his fear and killed the British Templar, but succumbed to his wounds after carrying the uncounscious Atsuko away from the battlefield with the last of his strength.
Atsuko was eventually taken away and had her wounds treated by Masajiro, in fact the Assassin and spy Matsuo. After she was told by him about her brother's deeds, saving her and the Murashi Masamune, Atsuko accepted Matsuo's offer to join the Assassins.
Behind the scenes
Historically Takeko is said to have been beheaded by her younger sister Yūko with the help of Ainu soldier Ueno Yoshisaburō.