Yue is a vampire who was imprisoned in the abyss. She was betrayed by her uncle and peers because of her monstrous magic and regeneration that made her nearly immortal.
With a fateful encounter, Yue met Hajime in a sealed room in the abyss. After being rescued, Yue was the person who suppressed Hajime's madness before he became a monster and she soon became Hajime's "special" person after what transpired in the Abyss.
Her appearance was described to be like the highest grade bisque doll with blonde hair and crimson eyes. She is 323 years old despite her small physique because she is a Vampire Princess. Her name 'Yue' isn't her real name and was only given by Hajime so that she could leave her dark past and move on.