A former Zi-Fighter and Amy's former lover, Kidd was soundly beaten in a championship battle and left the league. He returned with a Holotech Saber Tiger, an unusual Zoid that had the ability to turn invisible and conceal itself from sensory detection. Using the Zoid, he went on a rampage, attacking other Zi-Fighters. He was able to destroy many Zoids including two Blade Ligers, the Dark Assassins' Killer Spiner, and Sigma's Boldguard before being stopped by the Mach Storm team.

(Source: Wikipedia)

R.D. is a delivery boy who works for a company called Mach Storm in order to earn money to search for the legendary Alpha Zoid. A Zoid that he heard rumours about from his dad that he passionately believes in. Mach Storm also doubles as a Zoid Battling team. In R.D.'s first Coliseum Zoid Battle, he encounters a team that can fuse Zoids. R.D. soon discovers that he has a Zoid that can computably fuse with his Zoid, Liger Zero. After this happens R.D.'s adventure to discover the truth about the Alpha Zoid begins.
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(Source: Anime News Network)