Originally from the moon, her home was destroyed by Lunatyx, one of the oldest Choju. Most of her people were killed but some survived and wandered the galaxy as nomads. Yuuko returned to Earth to fight against Yapool, but her comrades in space eventually called on her to their new home. This was around the same time Lunatyx appeared on Earth, eventually revealing her true nature to Seiji, as Ultraman Ace the avenged the lunar civilization. Afterwards she left, donning a white garb, she left her Ultra Ring and her connection to Ace for Seiji, wishing both Ace and Seiji goodbye as she left the Earth. She is replaced by another crew member named Minawa Noriko.