Key is an artificial intelligence created by Cayna's uncle to assist and provide companionship after her accident confined her to a hospital bed. Originally functioning as a virtual assistant in the real world, Key supported Keina (Cayna) by executing everyday tasks. This included managing calendars, retrieving information, controlling smart home devices, facilitating communication, making phone calls, sending messages, obtaining directions, checking news or weather, and providing entertainment like playing music or games.

Following Keina's death and her transition into the game world of Leadale as Cayna, Key persists within this new environment. In the original novels and manga, Key manifests as a disembodied voice with no physical form. The anime adaptation, however, depicts Key as a small, floating green sphere visible exclusively to Cayna.

Within Leadale, Key retains its core function as Cayna's dedicated support system. It monitors for incoming dangers and remembers details on her behalf, a role Cayna calls her "watchdog." Leadale's native inhabitants consider Key a Divine Spirit and can occasionally perceive its voice despite its usual invisibility. The AI's name originates from a cat once owned by Cayna's mother. The English release of the novels refers to Key with male pronouns. Key's existence remains intrinsically linked to Cayna's journey, providing continuity between her past life and her current reality within Leadale.