Nagisa Ujibe is a former champion Keijo player who now serves as an instructor at the Setouchi Keijo Training School. During her youth, she rose to the highest professional rank in the sport, achieving the title of Prize Queen, and was known by the nickname the Siren. Her combat style was often described as captivating and reminiscent of a mermaid, and she was one of the early pioneers of the sport. Over time, her physical appearance changed significantly, and in her later years she carries a heavier build. Despite this, she remains a formidable presence, able to handle her students with ease during training exercises and quick enough to react to and evade advanced techniques such as Nozomi Kaminashi's Vacuum Butt Cannon.

Nagisa’s personality is serious, logical, and disciplined. She holds a strong sense of duty and responsibility, both as an instructor and as a former competitor. She is deeply committed to the integrity of Keijo and is not afraid to involve herself in the sport’s politics to expose corruption and ensure fair competition. She values honesty and ethical behavior above all, and she does not tolerate dishonesty from her students or opponents. Her motivations stem from a desire to guide the next generation of players and to maintain a balanced and fair playing field. She initially became an instructor out of concern that the power balance in the Keijo world was shifting too heavily in favor of the East, and she has since dedicated herself to training new talent.

In the story, Nagisa functions primarily as a mentor figure, especially to the protagonist Nozomi Kaminashi and her peers. She is the first person to recognize the potential and the danger of Nozomi’s signature Vacuum Butt Cannon, as she herself once used a similar technique. Her role extends beyond simple instruction; she challenges her students to push past their limits and relentlessly pursues their improvement. Her key relationships include that with Minamino, whom she mentored in the past, and with the main cast of trainees at the Setouchi school. Over the course of the series, Nagisa demonstrates growth in her willingness to trust and invest in her students, and she shows a more caring side beneath her strict exterior.

Notable abilities reflect her elite background as a former Prize Queen. She possesses exceptional reflexes and tactical knowledge, allowing her to analyze techniques quickly and counter them effectively. Even in her current state, she can easily keep up with younger athletes in practice matches. Her experience gives her deep insight into the mechanics of Keijo, and she uses this knowledge to develop training regimens and strategies. While she no longer competes professionally, her skills remain a clear indication of why she was once considered one of the sport's greatest stars.