Saitohimea, also known as Himea Saito, is the final survivor of an ancient race called Vampires, where "Vampire" signifies "Most Ancient Sorcerer" (最古の魔術師), marking her as a supremely powerful magical entity.

Her childhood involved relentless pursuit and experimentation by groups like the Church, drawn by her immense magical worth. Facing despair on a playground, she encountered Taito Kurogane, a human boy uniquely able to see her. This connection sparked deep affection, as he alone didn't seek to exploit her powers. To forge an eternal bond, she bit Taito, infusing him with her "poison," granting conditional immortality: he resurrects after death unless killed seven times within fifteen minutes.

Soon after, a magician later identified as Kurenai Hinata attacked them. To prevent Taito's permanent death, Saitohimea agreed to leave with Hinata on the condition that Taito's memories of her were erased. This led to her capture and nine-year imprisonment in a lightless, soundless extradimensional void without sustenance, demonstrating her Self-Sustenance. The crushing isolation intensified her fear of loneliness, unconsciously creating an alternate personality named Bahlskra as a coping mechanism.

When Taito regained his memories after a near-fatal accident, Saitohimea's powers surged, enabling her escape. She reunited with Taito and enrolled at Miyasaka High School as Himea Saito. There, she joined the student council, projecting a cheerful, affectionate exterior marked by physical embraces and unwavering loyalty towards Taito. Beneath this, she carried trauma from her imprisonment, including a specific fear of the sea associated with that captivity.

Her magical repertoire is vast: dimensional travel via portals, summoning magical beasts, levitation, vibration manipulation through shockwaves, light restoration, perception manipulation to avoid detection, and altering age or clothing at will. She possesses immense resistance to curses, poisons, magic, and existence erasure. She later shared the forbidden fire-manipulation technique "Vampire Flame" (貂魔の炎) with Taito, lethal to ordinary users.

Beyond her loving facade, complexities emerge. She exhibits jealousy over Taito's closeness with his childhood friend Haruka and fierce protectiveness, willing to become "someone's slave" for his safety. The isolation left deep psychological scars, manifesting as the unstable Bahlskra personality, which can surface under stress. She displays genius-level intelligence, evidenced by her precise calculation of her imprisonment's duration and extensive knowledge of interdimensional history and entities.

Her vulnerabilities include emotional dependency on Taito, exploitable by adversaries, and psychological instability stemming from her trauma. Her aversion to the sea and moments of intense jealousy further highlight her fragility. Despite enduring years of torture, she remains compassionate, reluctant to harm others.

Her role connects to a prophecy foretelling her potential as "Ramiel Lilith," a figure destined to destroy the world. This prophecy intertwines with the broader conflict involving the godlike Tenma and their messengers, the Angels (Tenshi). Her interactions, such as with student council president Gekkou Kurenai and his familiar Mirai Andou, illustrate her integration into human society while navigating her ancient heritage.

Relationships define her journey: her bond with Taito drives her actions, while rivalry with Haruka Shigure introduces emotional conflict. She forms tentative alliances with Gekkou and Mirai despite initial tensions, demonstrating her capacity for trust beyond Taito. Her past as an experimental subject and yearning for genuine connection underscore her narrative as a powerful being seeking belonging amidst isolation.