I also have to pitch in with the Jenova isn't evil crowd, with some more detailed headcanon about what happened between the Cetra & her. I've always had this idea about Jenova since I first saw the
appearance of Red Lifestream when Sephiroth dies in FFVII.
Reunion is more than just a cellular regenerative capability. It has to do with the very nature of how alien Jenova is as a lifeform. She's made of an energy that's so foreign that it is actually
incapable or returning to the Planet and joining the Lifestream. She travels huge distances in space via a cataclysmic meteor, with only minimal semblance of control over where she goes. When she arrives, the first thing she attempts to turn herself into people recognized by the locals and assimilate, but because it's their dead relatives, she's instantly shunned, and there's a conflict that starts over it. The Cetra found out that even if you obliterate Jenova's cellular material into Lifestream energy, it's so foreign that it can't merge into Gaia's lifestream, and coalesces back towards her body.
Their first attempt to defeat her involved injuring her greatly enough that they were able capture some of her lifestream energy and condense it into a Materia that they could use to defeat her. Instead of doing what they intended, it just solidified her own magic into a Materia that was useless to them, because as a weapon- it would kill the planet. That's where the Black Materia came from, and why it's meant to be used by channelling the energy from a planetary wound, and not weilded by a single person. Although it failed as a weapon, it allowed them to study the magic, and craft one that could oppose it - the White Materia - that they then kept to themselves and passed down from one generation to the next. The Cetra then took the Black Materia, and because of Jenova's shapeshifting magic, they were able to transform the Materia itself into
the Temple in order to lock it away. Unable to defeat Jenova, they sealed her away to prevent her from regenerating or summoning Reunion, but this is also why she/Sephiroth now needs the Black Materia to complete Reunion, because it contains the crystalized Lifestream that was taken from her, and she's incomplete without it.
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