Jenova

Jenova is an alien calamity that fell to the planet Gaia 2000 years before the events of Final Fantasy VII. Referred to as "Mother" by Sephiroth, and his remnants Kadaj, Yazoo, and Loz, Jenova's influence in FFVII makes her a major antagonist within FFVII's extended universe, Compilation of Final Fantasy VII. Experiments on her cells, mainly performed by the Shinra Electric Power Company, have produced many of the games antagonists, including the primary antagonist, Sephiroth.

FFVII
2000 years before the events of FFVII, Jenova landed on the Planet in a meteor, creating a large wound in the planet called the Northern Crater. Jenova emerged from the crater, and using her powerful genetic abilities, managed to nearly wipe out the Cetra, the cousin-species of the humans. The few surviving Cetra banded together and managed to seal Jenova in the Northern Crater, where she would lie dormant for thousands of years.

Jenova was eventually discovered by Shinra's leading scientice who labled her accidently as a Cetra and used her cells to create powerful super soldiers for their elite fighting force known as SOLDIER. During a mission in Nibelheim, an eminent SOLDIER, Sephiroth, discovered Jenova because he was told by Professor Hojo that his mothers name was Jenova Sephiroth came to believe that Jenova was his biological mother. After researching further about Jenova in an abandoned Shinra Mansion, Sephiroth reads, by mistake, that Jenova is a Cetra leading him to think that he is one as well. Angered by his belief that humanity abbandoned the Cetra to defend the world against an evil calamity, in fact Jenova, Sephiroth desires to avenge Jenova and the Cetra by destroying humanity. Sephiroth then murders all those in the town of Nibelheim and cuts off and takes Jenova's head with him, however he is defeated by Cloud and presumed dead. In actuality Sephiroth had joined the Lifestream and learned all he could about the history of the planet, he then finds an ancient spell that will sumon a meteor to create a wound in the planet.

Sephiroth comes to believe that he is the chosen one destined to rule the planet as a god, to do this he will sumon the meteor create a wound in the planet and as the planet tries to heal itself using the Lifestream he will absorb the Lifestream to become a god. While in the Lifestream Sephiroth decides that he needs a new body so he begins to make a new one, but in the mean time Sephiroth used his control over Jenova's remaning cells to make her take on his own form and act as his temporary avatar. While under Sephiroth's control Jenova was used to kill the heroin Aerith as she was trying to summon another spell, called HOLY, that would stop the METEOR one. When Sephiroth's new body had finally formed and he was about to become a god Jenova confronted Cloud and the others and fought them in her Jenova-SYNTHESIS form, but she was ultimatly defeated.

FFVII Advent Children
Later on Jenova's head was taken by Shinra's curent president Rufus Shinra and kept inside a box. As remnets of Sephiroth came into being as three young men Kadaj, Loz, and Yazoo they began searching for Jenova's remaing cells who they refered to as "mother." Kadaj in the end finally manages to recover Jenova's head and absorb her cells becoming reborn as Sephiroth, but he is once again defeated by Cloud.