Originally he needed to join his Ultra Ring with Yuko's to transform into Ultraman Ace after he merged his life force with both of theirs after they were killed in action. Later Yuko, revealed to be from the long destroyed Lunar civilization returned to wandering people after the defeat of Lunatyx, the beast responsible for her home's destruction, leaving Seiji the sole owner of the Ultra Rings and sole Host to Ace, though she still visit once in a while.
n episodes 33 & 34 he returns with Shin Hayata, Dan Moroboshi, & Hideki Go. The group gathered for a celebrative barbecue with Taro and Zoffy in human hosts. Unfortunately the Ultra Brothers' celebration was interrupted by Alien Temperor. For a time the Ultras switched hosts. Eventually they transformed and defeated the evil invader and returned home.