Marie Ornstein, born into a lower noble family entrusted with territory oversight, serves as Shion Ornstein's protective older sister. She possesses fiery orange hair with a slightly fluffy texture, typically styled with a braid on her right side. Her personality blends selfishness with innocence, matched by a strong physical aptitude that drives her dedicated swordsmanship training—motivated solely by safeguarding Shion.

Marie dotes profoundly on her younger brother, noticing subtle aspects of his behavior unseen by others, like solitary play or unexpectedly mature expressions. When Shion withdraws after discovering their world lacks magic, Marie responds by guiding him to a lake where they witness a luminous phenomenon resembling magic. This experience reignites Shion's passion for magical research.

Her protective resolve intensifies after a goblin invasion of their mansion, where she witnesses Shion in immediate peril. Haunted by the trauma, Marie trains relentlessly—sometimes injuring her hands—to ensure she can prevent future threats, cementing swordsmanship as her path to strength.

Marie’s emotional state directly fuels Shion’s early magical breakthroughs. His research reveals that strong positive emotions, particularly affection, manifest as a warm aura. Marie’s presence and support thus catalyze his initial success in creating magic, termed the "demon belt state." Their mutual confession of feelings deepens their bond, though it occasionally heightens Marie’s dependence on Shion.

She trains alongside peers like Rose, her relationships and experiences shaping her growth as she balances protective instincts with Shion’s expanding independence and magical pursuits.