Ясурі Нанамі

Though she appears to be helpless on the surface, she is actually stronger than Shichika. In episode 4, she single-handedly defeats three Maniwani head members without putting forth much effort. She's soft-spoken yet terrifying to face in battle and displays a critical lack of basic human understandings, much like Shichika did early on in the series.

Nanami's disregard for basic human values is even more severe than Shichika's. She flashes a very dark and creepy smile on occasions, hinting at a bloodthirsty nature that lies beneath her fragile exterior. As the seasons go by, her inner murderer gains more sway and she becomes noticeably insane and even fatalistic, in part due to the increasing suffering her body is undergoing. Deep down, all she wished for was to perish peacefully by learning Kyotōryu.

Despite not being taught her family's fighting style, she has keen senses, fast reflexes, and the ability to think ahead, as she had observed her brother during his training. Her unique ability, Perfect Observation, allows her to copy the strengths of those she fights. If she sees a technique once she can imitate it, twice and she masters it and can combine it with any other moves she has learned. She also has an unnatural ability to recover from serious injuries and poisons. However, her bad physical condition limits the amount of time she can fight at full power

Nanami comes into possession of Akutō Bita after killing off the people in Shireizan. The katana is later seen stabbed into her chest, which forcibly heals her physical weaknesses. She later confronts her brother and asks him to kill her so she could die and be free of her persistent suffering, a harsh side effect of Migeiko. In the end, she's killed by her brother, granting her the death she always wanted.

(Source: Katanagatari Wiki, edited)