Carmilla, formally titled the Witch of Lust, lived over four centuries prior to the main events. She began as an ordinary village girl in a small country, engaged and living peacefully until awakening the Authority of Lust. Her life shattered when a powerful man desired her, prompting her fiancé to lead a village rebellion. The conflict spiraled, toppling larger nations as Carmilla became an unwitting symbol of revolution. Her followers worshipped an idealized image, ignoring her true self. This culminated atop a conquered castle where her fiancé proposed publicly; she rejected him and abandoned her old life. The widespread wars and personal betrayals left her disillusioned and alone. She ultimately died at the stake in Lugunica, though conflicting accounts hint the Witch of Envy might have killed her. Her soul was later preserved by Echidna within the Castle of Dreams.

Physically, Carmilla stood 160 centimeters tall with light pink hair reaching her hips, typically tied in loose asymmetrical bunches. She possessed round, red downcast eyes and a face considered adorably gentle rather than strikingly beautiful. Despite a surprisingly shapely figure, she wore concealing clothing: a dark-red ankle-length dress with diamond patterns, light-brown boots, a long white coat featuring a pink collar, and an excessively long light-green muffler trailing behind her. This attire exposed only her head and hands, reflecting her aversion to showing skin.

Her personality exhibited extreme timidity, low self-confidence, and profound social anxiety. She frequently stuttered, avoided eye contact, and pleaded not to be harmed. This demeanor masked a fiercely self-protective nature; she held deep grudges against anyone who deceived or harmed her, regardless of justification, and prioritized avoiding personal pain above all else. Ironically, despite her title, past trauma left her with no desire for love, expressing disdain for being loved without genuine connection. She believed love itself was sacred and would not tolerate its denial. Her interactions with fellow Witches like Echidna and Typhon involved playful teasing mixed with underlying care, though she resented their manipulations.

Carmilla's Authority of Lust manifested primarily through "Faceless Bride." This power involuntarily captivated anyone who saw her, causing them to lose awareness of their surroundings and bodily functions—often leading to death by asphyxiation or cardiac arrest without their realization. She could also project illusions, appearing as the person her viewer most desired to see or making them hear a loved one's voice. Additionally, she could induce empathic understanding of emotions, such as revealing another's love to a target. These abilities were uncontrollable and perpetually active, necessitating bodyguards with "tough hearts" for partial resistance. Despite lacking combat skills, theoretical matchups suggested she could narrowly defeat Capella, the Sin Archbishop of Lust who later held her Witch Factor.

During Subaru Natsuki's trials within Echidna's Tomb, Carmilla played a critical role under Echidna's direction. Appearing during Subaru's mental collapse in the "present" trial, she used her Authority to assume the form and voice of Rem—the person he longed for most—to soothe his despair. She embraced him, offered unconditional love, and nearly kissed him before he recognized the deception, noting Rem would never encourage him to abandon others. Carmilla defended her actions as following Echidna's instruction to "be kind and dote on him," highlighting her power's duality: while it caused unintended harm, here it provided genuine emotional relief.

Following the events in Echidna's dream realm, Carmilla's soul was contained within a glintstone necklace worn by Omega, Echidna's artificial vessel, allowing her presence to persist beyond the Castle of Dreams.