Devilman vs Getter Robo is a crossover manga between Devilman and Getter Robo. Its Devilman characters combine elements from their TV-series and manga counterparts.

Hordes of demons, once thought entombed forever in the underworld, have now returned to Earth. Evil and corruption begin to seep into our world, as demons seize human hosts. Mankind’s only hope for salvation is to use the demons’ power against them–and only a pure-hearted man like Fudo Akira can do so without losing his humanity. Once little more than a crybaby, Akira now wields the terrible power of a devil, yet holds the innocent soul of a man–Devilman! 
 
(Source: Seven Seas Entertainment)

The first Getter Robo series chronologically. Published 15 years before the rest of the manga, Getter Robo pioneered the idea of the combining and transforming robot. 
Millions of years ago, the dinosaurs were driven deep underground due to the sudden arrival of "Getter Rays" on Earth. After evolving into sentient beings, they grew bitter and vengeful at the humans who replaced them on the surface and began plotting their attack on humanity. 
Fighting against them is Getter Robo, a machine built by Professor Saotome which uses the mysterious Getter Rays as its power source. Piloting it are three strong-willed young men - the violent martial artist Nagare Ryoma, the psychopathic yet highly intelligent revolutionary Jin Hayato, and the hot-blooded Judo master Tomoe Musashi. Each controls one of three specially designed combat jets which can be combined together in different forms to form three different kinds of giant robots: the flying, axe-wielding Getter-1, the quick, drill-wielding Getter-2, and the tough, tank-like Getter-3.

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