Yukino Yayoi, originally La Andromeda Promethium, is the Millennial Queen dispatched from planet La-Metal to secretly oversee Earth, preparing for her planet's return. Stationed on Earth for nearly a millennium, she adopts the human identity Yayoi Yukino, working as a private secretary for astronomer Professor Amamori at the Tsukuba Observatory. In the film, she also teaches at a school and resides in a high-class apartment, contrasting with the television series where she lives above a ramen shop run by adoptive elderly parents. Born in the Cocoon on La-Metal alongside her sister Selen and sent to Earth as an infant, she was raised by humans. Fascinated by astronomy and space, she earned a PhD and secured her position with Professor Amamori. By 1999, as La-Metal nears Earth on a collision course, she struggles between loyalty to her home planet and her attachment to Earth, particularly her adoptive family, Professor Amamori, and his nephew Hajime Amamori. Hajime, depicted as her student in the film, becomes a close confidant after discovering her true identity and choosing to support her. Her character arc centers on rejecting La-Metal's plans to enslave humanity or allow Earth's destruction. Witnessing human compassion and resilience, she defects from her mission, allying with rebels led by her sister Selen. This rebellion pits her against the Holy Queen Larela, her biological mother and La-Metal's ruler, who challenges her to a duel. In the film, Yayoi confronts Larela directly, defeats her, but refuses to kill her, retaining her title as Millennial Queen. As a La-Metallian and Millennial Queen, she possesses extended longevity, maintaining a youthful appearance for a thousand years, enhanced durability allowing survival from severe injuries, and rapid healing. Her crown grants access to psychic powers inherited from previous queens, including psychokinesis capable of halting natural disasters like tsunamis and clairvoyant visions of future events, though these cause intense headaches. She also coordinates Earth's defense efforts, including activating the Noah protocol—a massive underground vessel designed to protect humans during planetary disasters. Her relationships define her further. She feels deep affection for her adoptive parents and Hajime, viewing them as embodiments of Earth's virtues. She was once engaged to Dr. Fara, a La-Metallian, but their relationship fractures when he supports Earth's subjugation. Another subordinate, Daisuke Yamori, harbors unrequited feelings for her and eventually betrays her in the television series, though his role differs in other adaptations. After confronting La-Metal, Yukino Yayoi leaves Earth to resolve a black hole destabilizing La-Metal's orbit, sacrificing her ties to humanity. In later OVAs such as *Maetel Legend* and *Space Symphony Maetel*, her story continues: she returns to La-Metal, marries Dr. Ban, and gives birth to daughters Maetel and Emeraldas. To save her planet from freezing, she undergoes mechanization—a process manipulated by the scientist Hardgear—transforming her into a machine-dependent entity. This evolution leads her to establish the Machine Empire, enforcing mechanization across the universe and becoming the antagonist Queen Promethium seen in *Galaxy Express 999*. Flashbacks in later stories suggest she retained buried memories of her human experiences on Earth. Her final narration in the film encapsulates her duality: "I am Queen Millennia. I loved as a human... I rejoiced as a human... I grieved as a human... I will fade away... I am Queen Millennia."