Cossette d'Auvergne, an 18th-century aristocrat, fell victim to her fiancé Marcello Orlando—a painter consumed by the desire to eternally preserve her beauty. Following her murder, her soul remained imprisoned within a Venetian glass, imbued with the collective despair of her family’s lingering spirits and the malevolent aura of cursed household relics. After centuries of confinement, she encountered Eiri Kurahashi, Marcello’s reincarnation, binding him through a blood pact to atone for his predecessor’s sins by enduring agonizing physical and psychological trials.

Her relationship with Eiri unveils a duality: outwardly composed and cryptic, she exploits his devotion to exact retribution, yet grows increasingly conflicted as his torment unfolds. This tension intensifies with the emergence of a second entity—a mimic born from Marcello’s portrait—that manifests as a covetous, deluded replica of Cossette. Claiming legitimacy as the "true" heir to Marcello’s obsession, the imitation vies to supplant her, entangling Eiri in a web of rivalry and manipulation.

Over time, Cossette’s thirst for vengeance wanes. Confronting the repercussions of her schemes, she expresses remorse for entwining Eiri in her curse and resolves to dismantle the cycle of pain. Her redemption culminates in transferring her soul into Shoko Mataki, Eiri’s confidante, an act that severs her bonds to the glass prison and Marcello’s legacy. This rebirth grants her liberation, allowing her to transcend centuries of anguish.

Her appearance mirrors her tragic duality: cascading blond hair, piercing blue eyes, and a slender frame draped in lace-trimmed, ribboned gowns evoke both aristocratic refinement and spectral fragility. This haunting visage juxtaposes her morally gray choices, encapsulating a blend of calculated cunning and fractured humanity. Her arc intertwines obsession and absolution, charting a path from vengeance to self-sacrifice and ethereal emancipation.