Suzunagi was born during the late Edo Period in Japan, Suzunagi was born and raised in a Christian family, who happened to be indirect descendants of the Kaosu (The Ancient One) Clan. Members of the family (on her father's side) have the ability to predict the future with a 90% accuracy rate. Her father was a local playwright in the Edo area who ran a theater house (in the Japanese fandom, he's rumored to be Bushi, based on information found in the "Message magazine"). Her mother also helped with theater duties (it's rumored in the Japanese fandom that she may have also been a dancer of some sorts for the theater, as most believe she is the woman dancing on stage during the ending credits).
Suzunagi's father created and compiled a manuscript that would reveal to his audience the tales of five young men wearing magical suits of armors fighting the forces of evil. However, due to certain circumstances (some say it was because Suzunagi and her family were Christians, others say it was because Suzunagi's father was speaking out against the Shougun), the Shougun's samurai arrived and set the theater on fire. Suzunagi's father was then beheaded for his actions. During the chaos, Suzunagi's mother realized that her daughter was trapped inside the theater. While running inside to rescue her, Suzunagi's mother was killed when the roof of the theater caved in. Suzunagi was also killed in the fire.
Centuries passed and Suzunagi's spirit became a wandering spirit, unable to rest until the armors were annihilated. Because, in her mind, these armors brought about nothing but sorrow, pain, and death. While on her mission, she encountered Kaosu during the 1940s (more precisely, World War ll) as he was prepared to break the seal that prevented the Armor of Tenkuu (Armor of Strata) from falling into the wrong hands. At the time, Suzunagi learned all that she could about the manuscript her father wrote. She knew what was about to unfold and believed that the armors would unleash nothing but evil and corruption. Kaosu knew something was wrong with Suzunagi, as she was a girl filled with such anger and hatred. Suzunagi tried to fight Kaosu off, but Kaosu retaliated and released the Armor of Tenkuu to use its powers to chase Suzunagi away.
Many more years would pass before Suzunagi appeared once again, this time as an adult with more power and much more determination. With her abilities increased tenfold, she went after the very people that were involved in her father's manuscript: the Samurai Troopers. Originally she wanted to use the Troopers to summon their armors, the Kikoutei, and to use its power to "purge the world from the pain and sorrow, which the armors caused" (which would end up destroying the world, as well as Suzunagi, in the process). One by one, she captured the five young warriors by enticing them with their deepest fears and strengths. She had created her own set of armors that held the captive Troopers; however, she had no idea that her victims would become her saviors.
Suzunagi slowly realized that they suffered as well because of the armors. With their combined efforts, the Troopers helped Suzunagi overcome her pain and even be reunited with her late mother. Even without their armors, the Troopers gave her the strength to free herself of her inner demons. In return, Suzunagi gave them new and more powerful armors that were free from any influence of Arago or his evil essence. She also helped them realize that being Samurai Troopers didn't mean it was their destiny—it was a choice they would keep, as defenders of the Earth.
Like many of the OVA characters, Suzunagi does not appear in the manga.
Suzunagi's Japanese voice actress is Yayoi Mitsuki (Yayoi being her first name and Mitsuki being the last). Her English voice actress is performed by Canadian Ellen Kennedy. In the Italian dub, her voice is done by Sonia Mazza.