Kotaro Higashi wanted to be a championship boxer. However, fate had other plans as he fell thousands of feet from the Earth after Astromons, a new creature in the third age of monsters, shook him off. The Five Ultra Brothers brought Kotaro's body to their home world of Nebula M-78 many years into the past. It was there that Mother of Ultra merged Kotaro with Ultraman Taro, who would now form the Six Ultra Brothers. Before merging with Kotaro, Taro spent many years to hone his skills similar to the original five Ultra Brothers by mimicking their attacks through battles from both the past and future. After the merge was complete, Kotaro was taken back to present day Earth being the human host of Taro and as Astromons was ravaging a city Kotaro turned into Taro. Not long after Astromons's defeat Kotaro joined ZAT and fought in a new era of kaiju. Many foes were found that would threaten the Earth, but Taro and ZAT defeated them time and again with occasional help from the other five Ultra Brothers from the weaklings to monsters only Taro could defeat. After Samekujira attacked and Valkie fled Kotaro wanted to show the other Ultras he wanted to keep his humanity by no longer wanting to be Taro's host. Thus, he is one of the few Ultra human hosts to sever his relation with an Ultra Warrior.