Kei Sagara represents Petit Panini, where she works as a waitress, in a high-stakes martial arts tournament aboard a luxury liner. Standing 142 centimeters tall and weighing 39 kilograms, her figure measures 76-46-75 centimeters. She possesses a pale complexion, striking red eyes, and distinctive silver or violet hair styled in waist-length twin tails with a curtained middle part, often adorned with ribbons or hairpins. Her refined, stoic appearance features attire incorporating ribbons, scarves, puffy sleeves, and cross designs.

Trained in aikido for unarmed combat, Kei also wields supernatural lightning projection abilities, disqualifying her as a pure martial artist. Anticipating defeat in her first-round match against the reigning champion, Anna, she records a contingency video message. This message implores Yuu Asuka, the only person she believes capable, to intervene and stop Anna's destructive power should Kei lose. She battles knowing winners gain significant financial rewards and losers face severe consequences.