Mari hails from a lineage carrying a rare genetic condition, manifesting in striking ram-like horns and ultrasonic hearing. Her distinct traits provoked fear and prejudice among Numazu's townspeople, who branded her the "Demon Lord." Ostracized, she retreated into solitude within a Wassimer Island castle. Social anxiety and dread of further hostility kept her isolated, though she privately maintained a protective watch over the community.

Her existence revolves around familiars resembling sea creatures, with Pellapie functioning as her steadfast rotorcraft-like aide. Unseen by most residents, Mari covertly safeguards Numazu by detecting threats like unstable structures and relaying warnings to Dia via notes—actions that defy her fearsome epithet and reveal her innate guardianship.

Mari's transformation sparks when Yohane approaches without fear, expressing earnest curiosity and admiration for her horns. Yohane’s acceptance steadily erodes Mari’s defenses, guiding her through patient interactions to confront anxieties and build trust. Key moments include joining group activities, such as musical performances where she temporarily adopts a human-like form. These experiences fortify her confidence, diminishing her reliance on isolation.

Her bonds deepen profoundly: Yohane’s encouragement anchors Mari’s path to self-acceptance, while her once-distant ties with Dia and Ruby strengthen through shared efforts to protect the town. Despite progress, Mari’s reserved nature persists, reflecting the measured pace of her emotional growth.