R21, codenamed Sting, is one of four prisoners confined in a remote detention facility. She is the sole woman among the inmates, sharing captivity with a young man designated N17, a middle-aged man labeled N35, a girl known as R12, an elderly man called VO, and a cat. When an abrupt power failure cripples all security locks, R21 attains freedom alongside her fellow prisoners. Yet the complex reveals no trace of guards or viable escape routes. Her choices and maneuvers occur entirely within these sealed confines during an anomalous incident sparked by the old man's inexplicable conduct.

No data exists about her past, origins, history, or motivations before imprisonment. Likewise, no specifics are given regarding her growth, bonds, or experiences beyond this singular event. The character features solely in the original video animation "Five Numbers!" with no subsequent appearances or expansions in other official media.