Germatoid is an evil demonic monster who works for Pharaoh 90.
He possessed Doctor Tomoe following a lab explosion, thus making him evil throughout most of Sailor Moon S. Tomoe searched for Pure Hearts, at first to find the Purity Chalice, and then to feed Mistress 9, and once she awakened, she sent Tomoe after Sailor Uranus and Sailor Neptune. From here, Germatoid decided that he no longer needed Tomoe's body, so he left him, revealing himself to be a giant monster. Sailor Uranus blew him to bits, but he survived and entered several humanoid monsters created by Tomoe.
Sailor Neptune used her mirror to reveal the real one (as the others were fake), and Sailor Uranus stabbed Germatoid in the face with her sword, completely destroying him. With his death, the fakes all disappeared.