Koki Yamoto is a high school girl living in the cyberpunk metropolis of Neo-Saitama who becomes possessed by a powerful ninja soul known as the Shi Ninja through a phenomenon called Ninja Soul Possession. This traumatic event directly results in the death of her parents, leaving her deeply troubled and traumatized. She is an independent ninja who refuses to join the Soukai Syndicate, preferring to remain unaffiliated. By nature, she is reserved and introverted, and she cares deeply for her friends, often putting herself in danger to protect them. When she first gains the Shi Ninja’s power, she experiences a surge of violence and finds herself enjoying combat, which disturbs her. Over time, she gains better control over her abilities and does not let the aggressive impulses dominate her. Her primary motivation is survival and safeguarding those close to her, but she also struggles to maintain her humanity against the darker aspects of her ninja soul. In the story, she is introduced in an episode focused on her ordeal, and later becomes involved in conflicts with various ninja factions. She has a key encounter with Ninja Slayer during a battle against the ninja known as Sonic Boom, where she uses her origami-based powers to deliver the final blow. Although she is friendly toward Ninja Slayer and has received help from him, the Naraku ninja soul within Ninja Slayer holds a grudge against the Shi Ninja and demands her death, but Ninja Slayer spares her and allows her to escape. Her development is marked by her journey from a traumatized victim of possession into a more confident and controlled user of her powers, though the internal struggle with her violent tendencies remains. Her notable abilities are centered on origami manipulation; she can create and control paper constructs for offense and defense, most notably conjuring a large origami attack powerful enough to kill an enemy. When she taps into the Shi Ninja’s power, her eyes turn purple and a ghost-like purple scarf manifests around her, signifying her enhanced state.