Originating from the Underworld, Eucliwood Hellscythe—often called Eu or Yuu—belonged to the group known as the Seventh Abyss. While her specific past there remains undefined, she shared a close friendship with Naegleria Nebiros, who forged her distinctive silver armor and gauntlets specifically to suppress her immense magical power. She later resided in Tokyo for an unknown duration before encountering Ayumu Aikawa outside a convenience store. Ayumu attempted to befriend her but faced initial rejection. Shortly after, witnessing Ayumu's fatal attack by a serial killer, she intervened. Eucliwood resurrected him as a zombie guardian, an act which transferred the agony of his fatal wounds to her, forging their unique bond. Her reality-warping magic, capable of causing instantaneous death with a word like "die," is inextricably tied to her voice and emotions. Activating it inflicts severe physical pain upon her, compounded by her fear that her power would rage uncontrollably even after her death. To contain these risks, she suppresses her emotions, communicates solely through written notes—frequently detailing her substantial appetite—and wears her restraining armor constantly, even while bathing. Beneath her stoic demeanor lies profound compassion. She refuses to speak lightly of death, once slapping another character for its casual use, reflecting her intimate understanding of dying's pain. Intense self-loathing plagues her; viewing herself as a monster due to past accidental deaths caused by her powers, she would isolate herself after conflicts, blaming herself for endangering friends. Her character reveals gradual emotional opening. Initially perceived as emotionless, she later expresses care for Ayumu and others, confessing her love for him during confrontations with the declaration, "You are not the Ayumu I fell in love with." After defeating the rogue magical girl Kyoko—depleting nearly all of Kyoko's lives with a lethal word—Eucliwood gains incremental control over her magic, eventually speaking aloud without catastrophic consequences. Relationships anchor her existence: she considers Ayumu her servant yet acknowledges mutual affection. She lives with him alongside Haruna, a magical girl, and Seraphim, a vampire ninja who initially sought to serve her. Eucliwood maintains complex but caring ties, extending forgiveness even to antagonists like the King of the Night, a fellow zombie she resurrected. By the light novel's conclusion, she permanently loses all magical abilities, eliminating the necessity for emotional suppression or armor. She adopts a reclusive NEET lifestyle, implied to marry Ayumu while continuing to reside with their friends.