Wilhelmina Carmel serves as the Flame Haze titled "Manipulator of Objects" or "Manipulator of Ten Thousand Ribbons," bound to the Crimson Lord Tiamat. Tiamat's will resides in a divine vessel typically manifesting as a headband. Wilhelmina displays Dutch heritage traits and wears a practical purple maid outfit with puffy sleeves, white apron, yellow neck ribbon, petticoat, and knee-high boots, chosen based on her research into attire suitable for serving royalty; she wore a long yellow gown during the Great War.

Little is known of her background before becoming a Flame Haze, though Tiamat once called her "Princess," suggesting possible royal lineage. She fought alongside her close friend Mathilde Saint-Omer against the Crimson Denizen organization Töten Glocke during the Great War. Wilhelmina frequently battled Illuyanka but avoided confronting Merihim due to unrequited feelings. After Mathilde's death thwarting Asiz's ritual, Wilhelmina inherited Mathilde's final request: to find and train the next "Flame-Haired Burning-Eyed Hunter." This quest led to centuries of solitary travel, including a brief partnership with Flame Haze Rebecca Reed.

Centuries later, Wilhelmina discovered an orphaned infant identified as a "Great One" at Tendōkyū, a floating refuge for Flame Hazes. She became the child's guardian and trainer, adopting the maid attire for its practicality during duties like cleaning, laundry, and facility maintenance. She raised the girl rigorously but struggled to provide emotional support or life skills, partly due to her own lack of human socialization. Following Tendōkyū's fall, they separated. Upon reuniting in Misaki City, Wilhelmina disapproved of the girl's new name, "Shana," and perceived Yūji Sakai as a distraction from Shana's duties. This motivated multiple attempts on Yūji's life, driven by pragmatic concerns over the Midnight Lost Child treasure tool and an overprotective maternal instinct.

Her personality exhibits stoicism, formality, and emotional restraint, stemming from trauma over Mathilde's death and rejection by Merihim. She speaks in uncontracted, monotone Japanese, ending sentences with "de arimasu," often translated as "Indeed." Tiamat's vessel, Persona, manifests as a mask in battle, symbolizing her emotional suppression. Despite her aloof exterior, she harbors deep compassion, especially for Shana, whom she regards as a daughter. Her inability to cook becomes a recurring limitation, forcing reliance on instant meals—a contrast to her exceptional skills in singing, chores, and combat.

Character development occurs through interactions with Yūji's mother, Chigusa, who helps Wilhelmina acknowledge Shana's autonomy. She gradually accepts Shana's bond with Yūji, and her friendship with Chigusa deepens, exemplified by Wilhelmina creating a remedy for Chigusa's illness. During Misaki City's final battle, an existential crisis arises when Wilhelmina realizes Shana no longer needs her protection and learns friends Johann and Pheles excluded her from their plans. This resolves when they entrust her with raising their child, Justus, giving her renewed purpose to avoid past parenting pitfalls. Significant growth occurs when she finally addresses Shana by her chosen name before their parting, symbolizing relinquished control.

In spin-off media like *Shakugan no Shana-tan* and *Banjou no Carmel-san*, her character appears in comedic and super-deformed depictions, parodying her stoicism or domestic struggles. These appearances do not contribute to her canonical development.