Mariabelle is an immortal human and the assistant to Yūhi Shinatsuhiko, the district mayor of Sakurashin Town. She serves as Yuhi's secretary, and her existence is intrinsically tied to his power as a god, as his divine abilities are rooted in her. Mariabelle appears as a young woman with long blonde hair and blue eyes, possessing a slender, well-proportioned build. She is almost always seen wearing various cosplay costumes, and her facial expressions rarely change, giving her an outwardly calm and impassive demeanor. Beneath this composed exterior, she has distinct personality quirks, most notably a strong preference for Japanese food at breakfast, and she becomes noticeably upset if she does not receive it.

Mariabelle's background reveals that she is the daughter of V Calvani F. She is an undead artificial human, which grants her remarkable regenerative capabilities; she can recover from fatal wounds because she cannot die. When her throat becomes irritated, she occasionally smokes medicinal cigarettes to alleviate the discomfort. Her immortality and connection to Yuhi are central to her role in the story. At one point during a struggle with a character named Akane, Yuhi offers Mariabelle the opportunity to continue living with him. She accepts this offer, which restores Yuhi's god powers and establishes him as her guardian god, deepening their bond.

In the narrative of Yozakura Quartet, Mariabelle functions as a supporting character. Her primary role is to aid Yuhi in his duties as district mayor and to help maintain the stability of Sakurashin, a town where humans and youkai coexist. Her key relationship is with Yuhi Shinatsuhiko, whom she assists loyally and with whom she shares a profound connection. Her immortality and unique nature place her in a position of significance when conflicts arise that threaten the balance between the human and youkai worlds. Over the course of the series, Mariabelle's development is seen through her deepening commitment to Yuhi and her eventual choice to stay with him, solidifying her role as his guardian and companion. Notable abilities include her immortality and rapid healing, which make her extremely difficult to harm permanently. Additionally, her existence serves as a crucial anchor for Yuhi's divine power, meaning her well-being directly affects his ability to function as a god. This makes her both a valuable ally and a potential vulnerability. Despite her often emotionless expression, she displays loyalty and a quiet sense of duty, and her occasional moments of irritation or fussiness about breakfast hint at a more spirited personality beneath the surface.