Fuyumi Yanagi is a Japanese high school girl who becomes the central figure after she accidentally travels from the human world to the demon world through a mysterious portal known as the Black Curtain that appears in her bedroom. Shortly after her arrival, she is eaten by a carnivorous plant and transforms into a ghost, an event that sets the story's primary quest in motion. Following her death, she wears a distinctive white triangle-shaped headband, a common cultural symbol in Japan for denoting a ghost. She is described as being around sixteen to eighteen years old, with shoulder-length black hair and large dark blue or purple eyes.

In terms of personality, Fuyumi is portrayed as a kind, sensitive, and supportive individual, though she can also be somewhat naive and shy. Her nature is gentle and trusting, often finding herself surprised or quietly annoyed by the lack of common sense displayed by those around her, particularly the vampire Staz. Despite this, she develops an unquestionable faith in him, demonstrating a quiet inner strength and a willingness to stand up for what she believes in when necessary. She is known to be excellent at using proverbs to describe the strange situations she finds herself in and has a talent for making omelet rice. Her character serves as a thoughtful foil to the more outlandish and impulsive personalities in the demon world.

Fuyumi lives with her father in the human world, and her mother is notably absent. It is later revealed that her mother had a doppelganger in the demon world named Neyn, and the two merged upon meeting, which is why Fuyumi is not raised by her. This union makes Fuyumi the half-sister of the spatial magic user Hydra Bell, as well as of a character named Knell. Her primary motivation throughout the story is to be resurrected so she can return to her normal human life and family. She is determined to achieve this goal and works alongside Staz to find a way.

After becoming a ghost, Fuyumi faces a critical weakness: her spiritual body is an inefficient vessel for sustaining itself in the demon world. She requires a steady source of magical energy to avoid fading away entirely. This dependency forces her to rely on Staz, who allows her to drink a drop of his vampire blood to replenish her mana when she begins to disappear. The scientist Franken Stein theorizes that due to her unique nature as a ghost, Fuyumi possesses the potential to become a very powerful hybrid. She has the latent ability to absorb and utilize multiple types of magic from various demons, effectively using external sources like Staz's blood as fuel to grow stronger. This potential suggests that, despite her fragile state, she could one day rival even powerful demons in strength.

Her role in the narrative is that of the catalyst for the main plot, as the vampire Staz vows to protect her and find a way to bring her back to life after feeling responsible for her death. This quest drives much of the action, forcing various characters to confront each other and search for items like the Book of Human Resurrection. Over the course of her journey, Fuyumi grows from a passive and bewildered victim of circumstance into a more resilient individual who actively encourages Staz and contributes to her own rescue. She eventually comes to understand her complex feelings for Staz, realizing she has developed deep affection and trust for him despite the many dangers he pulls her into. Her primary desire remains to be resurrected and return to her peaceful, ordinary human life, which represents the normalcy she ultimately wishes to reclaim.