The scout for the invading Zigran race, Zigra began his attack on earth by blasting a moonbase and abducting the sole survivor, enslaving her mind and making her his slave. Zigra intended to conquer the earth, allowing his race to leave their own polluted seas to take over the comparatively clean oceans of Earth, before humanity can poison them with their own pollution. Worse, the Zigrans intend to use humanity as a food source! Gamera arrives and blasts Zigra's spaceship while it is underwater. The exposed alien soon swells to kaiju-size in the low-pressure Earth ocean and battles Gamera, freezing him with his paralyzing ray. Zigra then makes his demands upon the people of Earth, intending to make the nations surrender before he kills two small children trapped in a submersible. Gamera revives and rescues the children. Enraged, Zigra follows Gamera onto dry land, where the huge shark is soon overcome. Gamera fries the giant fish with his fiery breath, reducing Zigra to a charred corpse.

Zigra is a supremely confident monster. He displays no sense of caution, even pursuing his enemy onto dry land, despite his decided affinity for water. Although his immobilizing ray is highly effective, Zigra displays a preference for employing his razor-sharp fins and more physical attacks to using his beam weapon, perhaps indicating that he can only use it a limited amount of times before needing to rest and restore the beam's power. Zigra is a highly intelligent monster, able to communicate telepathically with other creatures, even humans.