Ciel, a light-hearted magus with singing abilities, serves as a spy for the organization Ideal. Her mission centers on locating "Histoire," with Ideal directing her to pursue another magus named Noir as the key to finding it. She possesses blonde or golden hair and wears a loose yellow cape, a black beret, tight white trousers, and gray wide boots.

Tasked with infiltration and intelligence gathering under Ideal's directives, she operates as an observer with strategic purpose. Her personality merges a cheerful demeanor with dedicated focus on mission objectives.

After her confirmed death, her body undergoes a memory transfer process. All memories of Noir from the inactive Mystere body are moved into Ciel's form, resulting in a fusion. This allows Noir to independently possess Ciel's body.

The fusion grants Noir, now inhabiting Ciel's body, immunity to the system-wide deactivation affecting other magus units across the planet and in space. Ciel's form creates a unique biological and technological exception to this universal shutdown.

Ciel first appears in the anime's second episode and performs an idol-like song in the eighth episode. Extended media such as spin-offs or movies do not explore further backstory or development beyond her role as a catalyst for Noir's evolution and the resulting immunity.