Hildegarde, known as Hilda, serves as the adoptive mother and wet nurse to Kaiser de Emperana Beelzebub IV, the Demon King's son. She cherishes this role as her primary purpose. Her appearance features glossy blonde hair typically secured in a bun, covering her enhanced left eye which detects objects ten times faster and perceives minute details beyond her right eye's capability. She favors an elegant Gothic Lolita style with black frilled clothing, though she adapts to other attire like school uniforms or traditional yukatas when needed.

Hilda maintains a cold, impassive demeanor, thawing only during interactions with Beel, toward whom she exhibits deep affection and fierce protectiveness. She diligently performs maternal duties including cooking, bathing, and nurturing him. Her personality includes a fondness for extremely spicy foods and a violent, bloodthirsty streak in combat, earning her the moniker "Oga's Bride" at Ishiyama High. She demonstrates strategic manipulation, orchestrating events to serve Beel's interests while possessing unwavering confidence in her combat skills. Her relationship with Tatsumi Oga begins with open disapproval over his perceived shortcomings as Beel's caretaker but shifts toward respect after witnessing his strength and dedication, particularly following his victory over Hidetora Tōjō. Despite this growing warmth, their interactions remain characterized by frequent bickering and mutual teasing.

Significant personality shifts occur during amnesia episodes triggered by Beel's kisses. In this state, she becomes notably innocent and affectionate: addressing Beel as "Beelze" instead of "Young Master," using Oga's first name, and displaying blushing with romantic hints toward him. This alter ego, interpreted as her repressed emotions, highlights underlying maternal and romantic feelings typically suppressed. However, she retains an instinctive disdain for Furuichi, viewing him as "a creepy douche," and shows jealousy toward Aoi Kunieda, whom she perceives as Oga's "other woman."

Her combat prowess is formidable, wielding an umbrella concealing a sword with agility and tactical acumen. During the Tōhōshinki Arc, she allows her own kidnapping to assess an enemy's strength before effortlessly dispatching his subordinates. In the Saint Ishiyama Academy Arc, she transfers schools to support Oga's group, manipulating delinquents into volleyball training and designating Kunieda as team captain after testing her skill. The Prince En Arc reveals her loyalty when she is impaled rescuing Kunieda from Hecadoth, a subordinate of Beel's brother En, showcasing her willingness to endure severe injury for allies.

Her background includes training at a demon wet nurse academy, where she fiercely competed with her sister Yolda for the top position. Before serving Beel, she attended to another demon figure, Iris. Her history with Yolda resurfaces during conflicts, such as when Yolda impersonates her and attempts to kill Oga, prompting Hilda's direct intervention, underscoring their tense familial dynamics.

Hilda's development reflects increasing integration into human world dynamics. Initially dismissive of human connections, her time with Oga and Beel fosters subtle changes, noted by other demons like Izabella who attribute shifts in her behavior to human influence. Her role evolves from a detached caretaker to an active participant in Beel's upbringing alongside Oga, balancing demonic duties with emerging personal attachments.