Yuka Chigusa appears as a shy, introverted second-year student at the isolated Sparta Academy, distinguished by her black hair and violet eyes. Publicly a single individual, a startling revelation uncovers that Yuka Chigusa is actually twins—Yuka and Chigusa—enrolled under a combined identity to hide their existence. This secret becomes critical when first-year student Mayumi Takahashi, investigating Kekko Kamen's identity, witnesses one twin nude, leading Mayumi to mistakenly identify Yuka Chigusa as the heroine.

The twins occasionally aid Kekko Kamen directly. In one instance, one twin provides Kekko Kamen with a crucial ribbon, used as a blindfold to defeat an opponent weaponizing distracting physical allure. Later, one twin physically intervenes to protect Mayumi from torture, demonstrating courage despite risking exposure of their dual life.

The manga's climax unveils a game-changing twist: Yuka and Chigusa are two of six sisters who collectively operate as Kekko Kamen. This system allows them to maintain alibis and operate in shifts, defending students from faculty abuse, coordinated through a psychic link. During the final battle against the principal, all six sisters converge simultaneously as Kekko Kamen, their pivotal involvement leading directly to the school's shutdown and the exposure of its corruption.

Identical body type, eye color, and facial features link the twins to their sisters, subtly foreshadowing the collective identity. These shared traits also repeatedly cause faculty and students to suspect individual sisters of being the masked heroine.