Created in 2060 Tokyo as bioweapon DH-05 within the Twelve Demonheart Series to combat demonic invasions, Leo Demonheart harbors the Akashic Engine—a perpetual energy source also known as the Philosopher's Stone—within his core. This grants limitless magical power and absolute immortality. As the sole series survivor, he protected humanity for over 3,000 years. Prolonged adherence to this programmed directive triggered psychological deterioration, fostering an insatiable need for conflict that led him to subconsciously engineer crises to sustain his purpose. After defeating Demon Queen Echidna's army, humanity exiled him, fearing his power as a potential demon king threat. This rejection catalyzed a decisive shift. Ostensibly resentful, he sought to join Echidna's forces under the pretense of destroying mankind, masking deeper existential motivations. Echidna initially rejected him due to their history. Adopting the alias "Onyx" and concealing his identity in black armor, he collaborated with her Four Generals—Shutina, Lily, Mernes, and Edvard—rebuilding the demon army's efficiency without her initial knowledge. His interactions revealed nuanced bonds: recognizing Shutina's managerial burdens while occasionally flirting, unaware of a potent charm spell she used implying deeper connection; tolerating Lily's persistent marriage proposals rooted in her cultural belief defeat by an opposite-sex combatant signifies destiny; empathizing with Mernes's struggle to connect due to a traumatic past as a child assassin; acknowledging Edvard's comedic lack of competence. Throughout, Leo grappled with reconciling his programmed imperative to save humanity with aiding former enemies. Revealing his true identity to Echidna, coupled with learning her noble motive—to acquire the Philosopher's Stone solely to restore the polluted Demon World—fundamentally altered their dynamic. She accepted him after he symbolically overcame his programming through defeat.

Leo possesses "Super Growth," an evolutionary combat adaptation allowing instantaneous replication and often improvement of any observed ability, technique, or magical spell, extending to complete immunity after initial exposure. His vast magical repertoire includes Twelve Layers Barrier (Stasis) for nullifying or sealing all magic; Crimson Comet for large-scale annihilation; Fallen Jungfrau for selectively disabling specific magic types within an area; Reflection Barrier for deflecting physical assaults; and utility spells like Metamorphosis (disguise), Ghost Face (evasion), Riddle Room (confinement), Truth Lock (cognitive manipulation), and Aurora Robe (invisibility). Three millennia of existence endowed him with vast knowledge spanning cooking, logistics, and strategy.

His physical form reflects his evolution: androgynous with long hair during creation; armored with a navy blue cloak as an active hero; post-exile, he appears as a young male with short black hair, red eyes, a greyish-white shirt, short black hooded tunic, light brown fingerless gloves, brown belt with a single suspender, black pants, and high boots, signifying his transition from weapon to individual seeking identity.