Episodes 25-26 involves Ryusei and Hakko teaming up with the Jack Blitzkrieg Cheer Squad, Hakko Kita's old biker buddies. The gang is played by an all-star lineup of veteran actors from the Japan Action Club, including previous Metal Heroes Hiroshi Watari (Spielban and Sharivan), Kenji Oba (Gavan) and Makoto Sumikawa (Spielban's Diana), as well as Sumiko Tanaka (the second Yellow Four) and Metalder's stunt actor Kazuyoshi Yamada.
Ken Uehara (Prof. Koga) was by far the biggest name in the show, having been a movie actor since 1939. (They might have been playing on this as a Casting Gag. Also, Akira Senoo was apparently very nervous upon meeting him - the debuting actor meeting the 40+ year veteran.)