Muteking, The Dashing Warrior

Twelve-year-old Rin Yuki loyally supported his father when the world laughed at the scientist for saying that Earth was about to be invaded from outer space. But Rin did not expect to become personally involved—until he met Takoro, the strange young "deputy sheriff" from another world who was on the trail of notorious space criminals. There were only four Kurodako Brothers, but they plotted to use their alien science and natural shape-changing powers to become the secret masters of Earth. With Takoro's help, Rin became the mighty super-hero, Muteking, to foil their schemes.