Yume Eguchi is a young girl transformed into the vessel for the powerful succubus Lilith. She possesses a petite stature, short purple hair reaching her shoulders with outwardly tilted ends, and dark blue eyes. Initially seen in a blue dress, she later dons the Tensou Academy uniform: a sailor cap and white one-piece sailor suit. When Lilith manifests, Yume grows a dark purple tail ending in a reversed heart-shaped symbol and wings formed from solidified demonic energy. The traumatic experience of becoming Lilith's vessel and gaining mind-control abilities subjected Yume to severe bullying, causing deep psychological distress and suicidal ideation. Her selfless nature turned this despair into martyrdom; she resolved against suicide to prevent Lilith from tormenting another host. She instead collaborated with the organization Taishikyoku, ostensibly aiding their plan to awaken and control Leviathan. Her true objective was betrayal, using Leviathan's power to eradicate both herself and Lilith. During the "Black Sword Shaman" story arc, Yume was confined within a Blue Elysium facility. Rescued by Sayaka Kirasaka, who sensed imminent danger, Yume was instructed to find Kojou Akatsuki. She located Kojou during his lifeguard duties, accompanied him, and briefly confused Asagi Aiba. After revealing her circumstances to Kojou and Asagi, she cried herself to sleep and was taken to a cottage for safety. New clothing was procured for her as she initially wore only a swimsuit. Yume's abilities derive from Lilith. As a succubus, she exhibits innate physical strength surpassing humans, demonstrated when her tail alone repelled Kojou and Yukina Himeragi. Her primary powers involve mental manipulation: - **Mind Berserk**: Amplifies a target's deepest desires to induce失控, ineffective against Primogenitors like Kojou. - **Mind Pacification**: Activated by humming a specific melody, emitting a yellow light from her body to soothe targets, potent enough to calm Leviathan. Her involvement includes visiting Saikai Academy to show Kojou her new uniform and participating in New Year temple visits with Asagi and Motoki Yaze. In the "War of the Original Vampires" and "Never-ending Night Banquet" arcs, her actions further intersect with the main plot. Long-term development reveals she eventually marries Kojou Akatsuki in the future, indicating integration into Itogami Island's social fabric and resolution of her earlier struggles, as part of a broader marital arrangement involving multiple characters.