Miku Isshiki is an 18-year-old Japanese female with brown hair, blue eyes, and a height of 1.55 meters. Orphaned alongside her older sister Yukari after their parents' deaths, they inherit substantial debts owed to a criminal syndicate. The organization specifically targets Miku for recruitment as a sex slave due to her youth, but Yukari negotiates to join her, seeking to provide companionship and protection.

Under trainer Shunji Iwashiro and psychological expert Takamura, Miku endures systematic training designed to transform her into a compliant sex slave. Unaware of their true situation, she mistakenly believes Shunji is Yukari's fiancé and their new life legitimate. This naivety erodes as training intensifies, incorporating public humiliation, enemas, and bondage.

A pivotal moment arrives when Miku loses her virginity during training, depicted through visceral imagery and a surreal sequence evoking psychological breakdown. Her mental state deteriorates further under coercive conditioning, culminating in a coerced incestuous encounter with Yukari, followed by a three-way interaction involving Shunji. These experiences fragment her psychologically, manifested through hallucinatory self-confrontations reminiscent of Neon Genesis Evangelion's abstract sequences.

Miku's arc concludes tragically. After Shunji rescues the mentally broken Yukari and is killed by the syndicate, Takamura assumes control of Miku. He erases her memories of Yukari, Shunji, and the traumatic events to repurpose her as an obedient slave. In the final scene, Miku inexplicably cries upon being informed of their deaths, indicating residual emotional trauma despite the memory alteration.

Her physical design resembles Asuka Langley Soryu from Neon Genesis Evangelion, particularly in facial features and petite stature.