Rin Asano was born in the 18th century, the only daughter to a master sword fighter. Her father was the latest leader and teacher of the Mutenichi-ryū school of swordsmanship. She had a happy, uneventful childhood. Which ended abruptly and violently when she was 14-years-old. Anotsu Kagehisa was building a new swordsmanship school, called Mutenichi-ryū. Rejecting the strict and rigid codes of older schools, his creation was based on adapting whatever combat method worked. Regardless how unconventional or dishonorable it was. He proved the superiority of his style by having his recruits assault and assassinate members of the traditional schools.
Rin stood witness to the assassination of her father and the subsequent gang-rape of her mother. She was not touched simply because Anotsu forbid his men to touch her. But neither he nor his men cared for her subsequent fate. She found refuge in an acquaintance for a while. But a single purpose dominated her thoughts. Finding a way to get revenge for her parents. She had some blade skills of her own and build improvised weapons. but this was clearly not enough against an army of killers.
So when 16-years-old, she located the immortal killer known as Manji and attempted to recruit him. When her money was not enough to convince him, she offered her body. Which angered him. He reluctantly took her assignment. Partly to gain a purpose for his immortal life, partly to maker sure that her revenge mission doesn't end up killing the "stupid" girl. He sees Rin as a surrogate little sister. She sees him as both a mentor and potential lover. Their path takes several strange twists and turns.