Shin Seiki Den Mars

A man with no memory, known as Mars, is found on a volcanic island during the Heisei era. He is actually an alien tasked with determining if Earth should be destroyed. Accompanied by the large robot Gaia and haunted by visions of a woman who tells him not to destroy the planet, Mars chooses to protect Earth instead, despite the conflict with his own people. If Mars dies, Gaia will detonate, causing Earth's destruction as well.