Cosette is a mysterious Trekker who once served as a feared operative connected to the organization known as Nocturn Eterna. After leaving that life behind, she joins the Boss's team and begins searching for a new purpose beyond violence and survival. Despite her intimidating reputation and combat abilities, her story revolves around redemption, trust, and learning how to live as an ordinary person.

Cosette has a striking and somewhat gothic appearance that reflects her Umbra, the Dark Magic of Nova World, affinity. She is recognizable by her long black-and-white hair, pale complexion, and mint eyes. Her clothing combines dark military and fantasy-inspired elements, giving her the look of both an assassin and a wandering hunter. She fights using a pair of steel claws attached to her arms, further emphasizing her dangerous and predatory image.