Getting it straight, it is “Giant Robo” who can be deemed as the rival of Ultraseven.
That’s another tokusatsu product which aired around the same time as Ultraseven from 1967 through 1968 on NET while produced by Toei Company (looks like it aired as “Johnny Sokko and His Flying Robot” in the US).
The product based on a manga series created by Mitsuteru Yokoyama was about a 30 meters tall giant robot named Giant Robo (GR1) as it is that was constructed by the BF (Big Fire), the secret alien organization plotting to conquer Earth.
While Robo is a robot which obeys a person who has initially recorded his voice on the control device shaped like a watch, Daisaku Kusama (acted by Mitsunobu Kaneko: 1957-1997), a team member of the United Nations Police Organization ‘Unicorn,’ got to record his voice onto the watch (although he looks too young to be the member).
Thus Robo ends up fighting against the BF in cooperation with the Unicorn members.
As Daisaku’s code name is ‘U7,’ it is said that it implies that the Toei people were fully aware of the presence of Ultraseven.
In a word, Giant Robo was a great product we children were irresistibly drawn to even no less than Ultraseven while featuring quirky and weird-looking monsters with totally a different feel from those of the Ultra Series.