Belle Hydra is a prominent character in the narrative, initially appearing as a powerful and enigmatic figure. She is a demon of the highest caliber, renowned as a top-level spatial magic sorceress. Her abilities in this field are exceptional, allowing her to manipulate the fabric of space itself. She is also known by the nickname Marshmallow, a moniker given to her by other characters, often in reference to her appearance.

In terms of background, Belle comes from the influential Hydra family and is the daughter of Heads Hydra. A significant revelation within the story is that she and the human girl Fuyumi Yanagi are revealed to be half-sisters, connected through their mothers. Belle is also the sister of Nell Hydra. Her role as a treasure hunter drives her to search both the demon and human worlds for rare and valuable objects, which is how her path initially crosses with the main characters.

Personality-wise, Belle is a complex character who can be difficult to read, often leaving others uncertain whether she is an ally or an antagonist. She possesses a confident and teasing demeanor, frequently engaging in flirty bickering with Staz Charlie Blood, the series' protagonist. Beneath this playful exterior, however, she harbors genuine romantic feelings for Staz, though he remains largely oblivious to them. This unrequited affection is a key driver of her actions, leading to moments of jealousy, particularly regarding his devotion to Fuyumi. Despite her powerful and sometimes ruthless reputation as a high-profile figure, she is also shown to have a more vulnerable and frustrated side, stemming from her hidden emotions.

Her motivations are closely tied to her personal desires and relationships. Initially, her interest in Staz is piqued by his sealed power, and she tests him by using Fuyumi as a bargaining chip. As the story progresses, her primary motivation becomes her feelings for Staz, which conflict with his singular focus on resurrecting Fuyumi. She eventually helps Staz on his quest, even providing him with the Book of Human Resurrection, but her actions are often colored by her personal stake in the matter.

Within the story, Belle serves as a crucial ally and occasional obstacle. She is the one who first meets Staz and Fuyumi after they use her gateway, known as the Black Curtain, which connects the demon world and Earth. Her spatial magic is indispensable for travel and information gathering, as she can create portals and observe events from afar. Later, she provides sanctuary for Staz and Fuyumi in her family's domain, Hydra. A pivotal moment in her role occurs when she and Staz engage in a fight to decide her cooperation; she ultimately surrenders before he can read her diary, which contains her true feelings for him.

Key relationships define much of her character arc. Her unrequited love for Staz is the most central, creating a love triangle dynamic with Fuyumi. Her relationship with her brother Nell is also important, as she helps Fuyumi escape from him when he tries to capture her on their mother's orders, leading to a misunderstanding with Staz. Furthermore, she develops a friendly rapport with Wolf, another main character, and is even mistaken for his girlfriend by his mother during their travels together. A significant development is her discovery that Fuyumi is her half-sister, which adds another layer of complexity to their rivalry and her feelings.

Belle undergoes notable development throughout the series. She transitions from a seemingly detached and manipulative sorceress to a more emotionally vulnerable individual. Her pride and attempts to hide her feelings for Staz are a constant source of internal conflict, but she ultimately becomes more open in her assistance, even if her pride prevents a full confession. Her journey also involves leaving her family home, effectively running away after the events surrounding Staz and Fuyumi, indicating a desire to forge her own path.

Her notable abilities are entirely rooted in spatial magic. She can create portals using any rectangular frame, such as a door or window, to travel instantly to any location or even between dimensions. These portals can also be used to spy on distant places, redirect both physical and magical attacks, and transport others against their will. One of her most formidable techniques involves creating her Grid Room, an isolated pocket dimension from which escape is nearly impossible. She can also use her power offensively by closing a spatial frame around a target, effectively dismembering them. Her ultimate spatial magic is known as the Black Curtain, which allows for travel between the demon world and the human world. While her physical strength is not her primary asset, she compensates with her cunning and the versatile, long-range applications of her powers.