Jenova
Jenova is a calamity from the skies, she is an alien life force that travels from planet to planet destroying them. Jenova was mistaken for a Cetra because of her vast powers when she was excavated by ShinRa. The scientist Hojo and Gast ran experiments and studies upon her. The documents on Jenova can actually be found in the Shinra Mansion in Nibelheim.
Jenova's cells along with mako are injected into SOLDIER members. This is probably due to the fact that she was mistaken as a Cetra and ShinRa wanted their fighting force to have the upper hand.