Aya Rindo

The protagonist, known as a detective specializing in monsters. She is an extraordinarily beautiful girl with large amethyst-like eyes and long black hair, possessing a youthful yet enchanting appearance. She is a unique being in this world, referred to as the "Immortal" monster, with an actual age of 962 years (physical age of 14 years and 3 months).

She had been living a secluded life under the Hasei family but was attacked by the Professor about a year before the story begins. During the attack, her head was severed by Jack's half-human, half-demon power. Although she did not die, she could not regenerate her body and remained as a disembodied head. Initially, she intended to have Tsugaru, who possesses demon powers, kill her, but after securing his cooperation to regain her body, she sets out for Europe, believed to be the Professor's stronghold.

As a disembodied head, she cannot move on her own and is usually carried around in a birdcage. She possesses extremely logical thinking and is quite knowledgeable due to her long life. Her immortality and small body (head) allow her to remain hidden in places inaccessible to ordinary people for extended periods. She has a bit of a gluttonous streak, but since she lacks a stomach, eating results in "something terrible happening," making it difficult for her to consume food.