Yuko Minami was once one of Ultraman Ace's two human hosts, along with Seiji Hokuto her close friend. The two became hosts for Ultraman Ace after they lost their lives trying to help a group of orphans get away from the first Terrible-Monster of Yapool, Verokron. When they bring their Ultra Rings together the two combined to form the giant of light, Ultraman Ace.

Originally from the moon, her home was destroyed by Lunaticks, one of the oldest Terrible-Monsters. Most of her people were killed but some survived and wandered the galaxy as nomads. Yuko 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 Lunaticks appeared on Earth, eventually revealing her true nature to Seiji, as Ultraman Ace they 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 Earth for the moon to take part in rebuilding the civilization there.