Nasuti aids the Troopers during their quest against the forces of evil. Not much is known about her past, except that she is half French and half Japanese, and was visiting her grandfather when Talpa's invasion began. Unlike her friends, Nasuti has no super powers or weapons. However, she is extremely knowledgeable about the mystical armors and various ancient artifacts. She provides vital information to the Troopers and acts as a caretaker for Ully. Over time, Nasuti becomes like an older sister to them.

In the beginning of the TV series, Nasuti is 17 years old. In Gaiden, she is 19 years old. During Kikoutei Densetsu (The Legend of the Inferno Armor) and Message she is in her early 20s.
During Kikoutei Densetsu, Nasuti was crucial in aiding the Troopers in decoding the mysterious language used by Mukara; and she was able to take herself and her friends to where Ryo and Sage were located.

In the 3rd and last OVA, Message, Nasuti was in New York City, where she was attending a special session of the United Nations General Assembly that was convened by the United Nations Security Council to solve the recent, bizarre events that have been happening worldwide; which was seen as a threat to international peace and security. During the meeting, Nasuti planned on telling the world officials about the existence of the Troopers. Whether she succeeded in doing so remains a mystery. Unlike the previous episodes, Nasuti only appears in the very last episode; but as always she provides support for Ryo who, at the time, was about to confront the new enemy, Suzunagi.
Nasuti's birthday technically falls on two dates, due to an error made by her creators at Sunrise Animation Studio. According to the YST book, Memorials, her birthday is listed on May 15, 1970, making her a Taurus. However, according to many other official YST merchandise, her birthday is listed as May 28, 1970, thus making her a Gemini like Seiji. Both official books were compiled by Sunrise; however, the latter date is often used in other official documents. It is also the most popular birthday used by fans.

Nasuti's family name, Yagyū is based on the real Japanese clan with the same name. Like most of the characters in the series, many of their surnames are based on historical Japanese clans.
Nasuti plays the same role in the manga as she does in the anime version of YST. She provides assistance to the heroes but unlike the anime, her importance is downplayed in order to make room for more action. Jun, the little boy who counted on the Troopers and Byakuen (White Blaze) to protect him, becomes more of a key figure.