Nasuti Yagyu is the granddaughter of Professor Konosuke Yagyu, a historian focused on legendary armors and the demonic Youja threat. Serving as his university research assistant, she possesses deep knowledge of these subjects, making her one of the few aware of the dangers. Her mixed French and Japanese heritage comes from her Japanese father and French mother; she spent her early childhood in France before moving back to Japan. At the series start, she is 17 years old.

When the Youja invade Shinjuku, Nasuti rushes to the city alone, encountering the five warriors and a young boy named Jun. She becomes their key source of intelligence and logistical backing. Her computer expertise and access to her grandfather's research files are crucial for locating the warriors after Arago's forces scatter them early on. While initially vulnerable to capture and used as a hostage in several early confrontations, she adapts by learning to avoid combat zones, significantly reducing this risk after approximately the twelfth episode.

She provides practical support utilizing her personal Mitsubishi Pajero Sport Turbo for transportation across Japan. Holding a French International Driver's License allows her to operate the vehicle legally despite being under Japan's minimum driving age. She also offers the warriors sanctuary at her family's secluded vacation home, providing them respite and normalcy.

A pivotal moment occurs when Youja forces kill her grandfather at the university; poisoned during an attack, he dies in her arms. Despite profound grief, she demonstrates resilience by continuing to decipher his research, uncovering clues about the warriors' armors' origins. Her emotional strength and lack of corruptible desires, shared with Jun, later enable them to locate and empower the mystical Inochi no Magatama without succumbing to Youja influence.

In the later OVA *Message*, Nasuti appears in her early twenties. She travels to New York City for a special United Nations General Assembly session convened by the Security Council to address unexplained global disturbances perceived as international security threats. During this session, she intends to reveal the warriors' existence and role to world officials, though the outcome remains unresolved. Her role in *Message* is primarily supportive, confined largely to the final episode where she aids Ryo before his confrontation with the entity Suzunagi.

Her character design includes a preference for functional clothing like culottes and skirts, chosen for comfort and mobility during field investigations with her grandfather. Her family name, Yagyu, references a historical Japanese clan.