Ciel is a central figure in the narrative of Synduality Noir, introduced as a charming and mysterious Magus, an artificial humanoid companion designed to assist Drifters. She is recognizable by her distinctive appearance, which includes long blonde hair, a black beret, and a flowing yellow cape, paired with a practical outfit of white trousers and wide gray boots. Her most defining trait is her love for singing, a talent she possesses naturally and which is her primary method of self-expression and emotional connection with others.

In terms of personality, Ciel initially presents herself as lighthearted and carefree, often appearing as a friendly and supportive figure who is willing to help those she meets. This warm exterior, however, conceals a more tragic and complex interior life shaped by her circumstances. She arrives in the story without a known master, wandering from place to place in search of her ideal partner, a quest driven by a deep-seated longing for a genuine and kind connection.

The truth of her background reveals her to be a spy, working on behalf of a powerful entity known as Ideal, with the specific mission of locating a key figure or item called Histoire, and she operates under the belief that her fellow Magus, Noir, is instrumental to that goal. Her true master is a cruel and paranoid individual named Weisheit, who lives deep within a hazardous ruin and harbors an intense distrust of all Magi, viewing them as unreliable tools at best. Under his control, Ciel was forced to perform various unsavory tasks, and whenever her emotional distress became too great, Weisheit would simply erase her memories, trapping her in a cycle of servitude and psychological torment.

Her role in the story evolves significantly after she crosses paths with the protagonist, Kanata, and his Magus partner, Noir. In one notable incident, Ciel is tricked into signing over her deed and is forced to participate in a battle royale in a gambling den. Kanata and Noir come to her rescue, winning her freedom. However, Kanata chooses to cancel his contract with her, allowing her to continue her search for a master of her own choosing, an act of kindness that deepens her bond with the pair. Her key relationships are therefore fraught with tension and tragedy: she is an abused subordinate to the villainous Weisheit, while she forms a genuine and caring friendship with Kanata and Noir, a connection that eventually inspires her to rebel against her programming.

Ciel undergoes a profound, albeit tragic, development. Over time, her feelings for Kanata grow beyond mere friendship, and she begins to act on them, often stepping in to provide him with emotional support and companionship in Noir's absence. This culminates in a final act of defiance, where she chooses to protect Kanata from Weisheit rather than obeying the order to kill him. In a desperate escape attempt, she utilizes her unique singing-based abilities to fight off her pursuers. Her rebellion is cut short when Weisheit remotely activates a device to erase her memories once more. Although she had installed a counter-program to resist him, the strain proves fatal. She dies in Kanata's arms, leaving behind nothing but an empty shell and a pre-recorded message of her singing.

Ciel does not possess conventional combat abilities. Instead, her most notable power is her voice. Through singing, she can produce potent effects, an ability that has been likened to the powers seen in series such as Macross, allowing her to resolve conflicts in a unique manner distinct from standard mechanical combat. Her legacy endures after her death. Following her passing, Kanata transfers the memories of Noir into Ciel's now-inert body. In doing so, Noir gains a new, independent vessel free from her previous limitations. As a result, Noir not only continues to live but also inherits Ciel's final, heartfelt wish: to sing. This allows Noir to perform Ciel's songs, keeping her spirit and her voice alive throughout the rest of the journey.