Rein is a centuries-old rasetsu (evil spirit) born from accumulated negative energy during Japan's Sengoku era. She was never human. Her true form is an 9 feet tall hourglass figure, elegant, oiran-inspired entity with pale skin, ornate hair, oshiroi and Beni make up, spider lily motifs, and a red kimono, paired with lifeless black eyes and sharp, teeth. In the modern world, she possesses a 17-year-old girl named Keiko after consuming her soul. Rein uses the body freely to blend into society, while secretly indulging in violence and manipulation for her own entertainment. Despite her sadistic and self- centered nature, she is curious about modern life, enjoys technology, pranks, motorcycles, and observing humans. She can be oddly soft toward children, and she has a strange weakness: cats make her unexpectedly gentle even in her monstrous form. she has rotting flesh tentacles that can retrack from her back at her will.