Mephistopheles, better known as Mephi, is an exiled succubus queen trapped in a human body. Once a powerful ruler over all succubi in Hell, Mephi was a force to be reckoned with—arrogant, self-obsessed, and capable of making anyone fall for her with a mere snap of her fingers. Her regal charm and eloquence commanded respect, but her downfall came at the hands of humans.

Her story begins when a human corporation, desperate to solve the world’s energy crisis, summons her to Earth with the intention of harnessing demonic energy. In captivity, Mephi is fitted with a collar that drains her powers whenever she tries to use them, rendering her weak and fragile. After enduring countless torturous experiments, she eventually escapes the facility, finding herself in the middle of winter in the same city as Dami and Yuki.

Unable to remove her power-draining collar, Mephi resorts to charming a radio tech store clerk, who becomes her unwitting savior by removing the device. Freed from the collar but still stuck in her human form, Mephi is now trapped on Earth, cut off from her powers and unable to return to Hell. Her ultimate challenge lies in reconciling her new reality: a once-mighty demonic entity, now powerless and exiled.

In her efforts to regain her former glory, Mephi goes undercover as an exchange student at Dami’s school. Though she befriends Dami, the latter remains unaware of Mephi's true identity, instead assuming that Mephi is suffering from a severe case of chuunibyou. All the while, Mephi secretly uses what little power she has left to feast on human energy, searching for a way to return to Hell while maintaining the illusion of being just another student.

Mephi’s journey is one of pride and survival, as she learns to navigate life in a world where her former power means nothing. Her struggle is both internal and external—adapting to her new life while plotting her eventual rise back to power.

Mephi’s hobbies are rooted in her former life as a powerful succubus queen, though she would never admit to indulging in such "human" pastimes. Secretly, she enjoys tabletop miniature demon armies, allowing her to feel like a queen commanding legions once again, albeit on a much smaller scale. However, she never goes as far as purchasing an army for herself. When walking through the city, she often pauses to observe tabletop shops through the windows.

She also finds interest in chess, relishing the strategy and control it represents. To Mephi, each piece symbolizes her influence, but the absence of true power behind the game sometimes leaves her frustrated.

In private moments, Mephi keeps a diary. Despite her arrogance, she writes down her deepest thoughts and vulnerabilities, though she is incredibly shy about revealing this side of herself. It remains her most guarded secret, hidden even from those closest to her.

Mephi stands at 147 cm with striking red eyes and blonde hair, accented by two red strands in her human form. She dresses in fashionable black outfits—whether elegant dresses or trendy streetwear—to enhance her charm, using her appearance as a tool to lure those around her, ever the succubus.

In her true succubus form, which she reveals only when feeding or needing her powers, Mephi transforms with white hair, two red demonic horns, and fangs that show her demonic nature. She often wears a bat-shaped bra and panties or gothic dresses, paired with black stirrup thigh-highs. Every movement Mephi makes is laced with grace and seduction, an innate ability she uses to captivate anyone in her presence.