Orphaned at a young age, Naoto was found suffering from amnesia after the extremely violent murder of her parents, which also resulted in an iconic X-shaped scar across the middle of her chest. She was taken in, raised and trained by a swordsman whom she believed to be her parents' murderer. He named her Naoto after someone else in his own past, but never revealed his own name to her. After his subsequent murder she discovered that her parents were killed by an assassin whom she was named after; the original Naoto. Following his death she has uncovered the name of her mentor, Fuyumine, and adopted it as her family name. It seems that after the years spent with Fuyumine she developed some affection for him and even refers to him as "father" after his murder. Since then Naoto made friends with Nill, and has been following Haine in an attempt to find more about Fuyumine's history, the first Naoto, and to have her revenge. She fights with the katana that she inherited from Fuyumine. Out of the main group of herself, Badō, and Haine, she seems to be the most gentle and least willing to kill.

She doesn't seem to get along very well with Haine and most of their time together away from fights is spent glaring at each other maliciously. This is probably due to the fact that Haine employs extemely brutal and cruel tactics during fights. Oftentimes, Badō has to ease the tension between the two. However, during confrontations with enemies her and Haine are shown to be lethally effective when working together.