Vermeil stands as the central female protagonist across all official media. Her origins stretch back over 550 years to Ortigia, where she lived as a young demon child in a monastery, cared for by a nun alongside three other orphans. This peace shattered when superstitious villagers, blaming her for a plague, executed her adoptive family before her eyes. Overwhelmed by grief and rage, she annihilated the entire city, an act that marked her as "the greatest disaster in history" hunted for two centuries. Ultimately, seeking an end, she requested her only friend Fatema seal her within a grimoire, hoping future liberation might bring acceptance and love.

Physically, Vermeil manifests as a tall, curvaceous woman with violet shoulder-length hair and vermilion eyes. She displays distinct demonic features: horns, pointed ears, and a spade-tipped tail, traits she can conceal to pass as human. Her typical attire is a short black dress, later replaced by a female student uniform after combat damage. Under extreme stress, magical coercion, or berserk rage, she fully transforms: growing black wings, six ears, furred goat-like legs, enlarged horns and fangs, claws, and a distorted face. This demonic state significantly increases her size and power but severely impairs her control over destructive magic.

Her personality blends bold flirtatiousness with deep emotional complexity. She exhibits a carefree, provocative demeanor, openly expressing physical desire toward her summoner, Alto Goldfield, showing little regard for modesty or social boundaries—entering male-only spaces or initiating intimate contact. This includes regularly draining Alto's mana through prolonged kissing and teasing him about more efficient methods like sexual intercourse. Beneath this exterior lies profound trauma and guilt over destroying the village, fueling intense self-loathing and fear of rejection. Her relationship with Alto evolves from contractual obligation to genuine romantic love, characterized by mutual protectiveness and declarations of commitment. This bond helps her confront and gradually heal her emotional wounds.

Vermeil possesses exceptional power even among demons. Her immense magical strength can level cities uncontrollably when emotionally overwhelmed, matched by superhuman physical force capable of repelling massive creatures effortlessly, and rapid regenerative healing. Her abilities include shapeshifting to hide non-human features, mana absorption via intimate contact (kissing or implied intercourse), and mana amplification to enhance Alto's capabilities. A pivotal development occurs when she shares part of her demon heart to resurrect Alto after accidentally impaling him during a berserk episode. This act creates an irreversible life-link—if one dies, the other perishes—and initiates Alto's gradual transformation into a hybrid demon-human entity. Her natural longevity spans centuries, though this lifespan becomes interdependent with Alto's after their heart connection.

Her narrative arc follows significant development. Initially summoned by Alto to avoid academic failure, their dynamic shifts from master-familiar to romantic partners through shared adversities, including battles against antagonists like Obsidian and Iolite. Her journey focuses on reconciling her destructive history with her desire for redemption, profoundly influenced by Alto's unwavering acceptance. Key moments include resisting Obsidian's control through loyalty to Alto, confronting traumatic memories via Iolite's magic, and pledging to stay with Alto permanently after encouragement from Lilia. Throughout all media, she balances carnal appeal with emotional depth, culminating in a committed interspecies relationship driving her ongoing growth.