Yukino Yayoi, also known by her true name La Andromeda Promethium, is a central figure in the narrative. Born on the Cocoon with a sister named Selen, she was sent to Earth as an infant by the absolute ruler of planet La-Metal, Holy Queen Larela. On Earth, she was adopted by an elderly couple who named her Yayoi Yukino and raised her above their ramen shop, where she assisted with deliveries. She leads a dual existence. Publicly, she works as a secretary for an astronomer, Professor Amamori, and in one adaptation also serves as a school teacher. Secretly, she is the current Millennial Queen, an operative from La-Metal who leads an underground colony of her people on Earth and has been preparing for her home planet's next approach. The core conflict emerges as La-Metal's orbit brings it on a collision course with Earth, threatening destruction. As the Millennial Queen, she is initially expected to facilitate her planet's plans, which involve abducting and enslaving Earthlings. However, having grown up on Earth and formed deep attachments to her adopted parents, friends, and the planet itself, she experiences a profound internal struggle. The love she received from humans conflicts with her duty to La-Metal, causing her to question her mission and ultimately rebel against Holy Queen Larela's destructive plans. This rebellion places her in direct opposition to her home world. She aligns herself with Earth's defenders, including Professor Amamori and his nephew, Hajime, a junior high school student who becomes a close friend. She also finds an ally in her long-lost sister, Selen, who leads a rebel faction on La-Metal. She is forced to confront her mother, Larela, in a duel for the fate of Earth. Though she emerges victorious, she chooses not to kill Larela, instead seeking a peaceful resolution. She is defined by a strong sense of justice, compassion, and a willingness to sacrifice herself for others. She possesses superhuman abilities, including great strength, the power of flight, and the capability to summon a powerful energy beam. Despite her reserved nature and initial reluctance to reveal her true identity, she demonstrates strong leadership and strategic skills in rallying allies to protect Earth. Following the confrontation, she journeys to La-Metal to correct a black hole problem affecting its orbit, ensuring the safety of both planets. This requires her to tearfully say goodbye to her loved ones on Earth, including her foster parents, Professor Amamori, and Hajime, to find a lasting peace. Physically, she is depicted as a tall, beautiful young woman with long hair and striking eyes. Her attire varies; she is seen in casual Earth clothing like jeans and shirts, a uniform for her delivery work, and her formal La-Metal royal regalia, which includes a black dress and a distinctive red crown.