Ultraman: The Ultimate Hero.

The members of WINR (pronounced "Winner") respond when members of the alien Baltan race attack Earth, but the Baltans are only fended off when a gigantic alien, Ultraman Powered, joins with WINR member Kenichi Kai and gives him the power to metamorphose into Ultraman in times of danger. At the end of the first episode Ultraman declares that the Baltans were not completely defeated and that he will remain on Earth to continue the fight. WINR and Ultraman destroy numerous other monsters, but WINR learns that the Baltans were arranging all the battles to learn Ultraman's strengths and weaknesses