Queen Promethium originated as Yayoi Yukino, born on La Metal and serving as Earth's Millennial Queen, secretly governing every thousand years per her people's tradition. Her human form possessed long blonde hair, a willowy figure, and brown eyes, typically dressed in black with a red crown featuring heart-shaped ornaments that later evolved into gear designs. This era revealed a dedicated ruler valuing life, who defied orders to destroy humanity to preserve La Metal's resources, demonstrating initial compassion.

Facing extinction from La Metal's failing artificial sun, Promethium initiated mechanization to save her civilization, tasking her husband, Dr. Ban, with creating machine bodies. Her daughters, Maetel and Emeraldas, opposed this path, but Promethium insisted. During transformation, scientist Hardgear interfered, corrupting the process. Dr. Ban attempted to merge his consciousness with hers via nanomachines to guide her, but failed. As her humanity faded, Promethium used her final moments of free will to dispatch her daughters to safety aboard the Galaxy Express 999. Hardgear tried to assimilate her power, only to be absorbed himself, accelerating her descent into tyranny and the establishment of the Machine Empire, enforcing mechanization as eternal life.

Post-mechanization transformed her physicality. Her primary machine body featured a pale, Noh mask-like face with entirely black eyes, a black veil-like form resembling deep space, a central red forehead module, and tendrils extending from her head. She sometimes wore a beaded tiara. Manga depictions presented her as a large, dual-faced head resembling both Maetel and the Noh mask, symbolizing her Janus-like role as a gateway to mechanization. Animated versions occasionally showed a more human-like appearance with blue hair and a black cloak. She could project holographic disguises of her original human form to manipulate others, such as luring Maetel back to La Metal in *Space Symphony Maetel*.

Her personality shifted from compassionate to coldly utilitarian, viewing organic life as inferior "foolish flesh-and-blood bipeds" and mechanization as evolutionary perfection. She enforced a hive-mind network on subjects to eliminate rebellion and secrets, conscripting them into labor. Despite ruthlessness, she retained a twisted affection for Maetel, seeing her as the only worthy organic successor. She implanted Maetel with a monitoring device to share thoughts and emotions, reflecting control and lingering maternal connection. Her arrogance in machine superiority dismissed organic resilience as a threat, contributing to her downfall.

As Machine Empire ruler, she wielded extensive abilities. She transformed humans into "Living Screws"—cybernetic entities powering her planets—via atomic restructuring triggered by lightning storms. She accessed and rewrote souls using "Soul Rings," data-based personality manifestations, creating loyal soldiers or android clones. She controlled all imperial technology and military forces centrally, projected planetary-scale illusions, and self-replicated consciousness into new bodies for near-immortality. Defensive capabilities included energy bolts and bell-shaped barriers. However, reliance on hive-mind surveillance created vulnerability during distraction, exploited by coordinated rebellions.

Her reign involved multiple conflicts. After Maetel and rebels severed La Metal's power core, her body disintegrated, but she transferred consciousness to a new artificial planet, Planet Promethium. Temporarily disabled by the boy Nazca's sacrifice destabilizing her energy system, she rebooted years later. In her final confrontation, she convinced Maetel to bring the boy Tetsuro Hoshino as a replacement Living Screw. Maetel feigned compliance but used Dr. Ban's preserved consciousness capsule to trigger an energy overload, destroying the Great Mother Planet and all regenerative systems, ending her permanently.

Her name derives from the radioactive element promethium, a synthetic material used in atomic batteries, reflecting her mechanical utilitarianism. It also alludes to Prometheus, who gave fire to humanity—paralleling her grant of mechanized "eternal life" at autonomy's cost. The Janus-inspired dual-face motif signifies her role as a gateway between humanity and machinery, embodying beginnings and endings.