Hey there I am back.. I have done a couple more art pieces since
I still adhere to the fact jenova comes from a cosmic virus.. and thus I did an image of that:
As well as an image of it sucking up the lifestream from the planet before it came to gaia:
Where the planet tries to activate its own holy materia but it is too late. On that planet tentacles beings are the dominant life as you can see a tentacled being on the far right has summoned holy - it's version of the "cetra" I suppose so this jenova looks like a tentacle creature with tentacles that can morph into spears and its arms mutating in a "the thing" like way, vacuuming up the lifestream beneath it.
My theory is that the black materia was created by the planet to use in conjunction with Omega - when a crisis faced the planet it would summon meteor to destroy the rest of the inhabitants and then summon omega to vacuum them up and bring them to another world. Jenova could in itself even be an omega weapon from another world that became corrupted and went from planet to planet liek a cancerous call to suck up their lifestream, evolving in the process, but i digress.
My theory is that a cetra - a female form the looks of the fresco in the temple of the ancients, for whatever reason used the black materia to summon meteor - perhaps she wanted to become a goddess like seph, and summoned the meteor, which was a fragment of a planet jenova was using to sail the cosmos - summoning it to gaia and causing the calamity - it took on her form or took over her body and went around morphing as other beings - much like sephnova uses sephiroths favoured body form in the game. Might explain her wings - as it might have been a composite being like sephiroth eventually became with jenova - part corrupted cetra goddess, part jenova shapeshifter, intent on destroying all and evolving. The ancients had knowledge of what the black materia could do - and they made it so that all were male except for one female, implying it was once upon a time used.
As for the eternal question of is it jenova or sephiroth as the villian - it is stated her cells are braindead - thus whatever goddess like being that kinda mirrored the sephiroth events before are isolated to that battle that had shiva and the knights of the wound (possibly) aka cetra back then contain her - but that this whole process was REPEATED with sephiroth - the braindead cells still were functional individually, so when implanted in a womb now had a new "brain" to replicate - much like taking a heart - stripping it of its entire cellular structure so its white and repopulating it with anothers cells to form a heart that isnt rejected by the body. Jenova took over sephiroth. Sephiroth never existed - it is jenova reborn, mimicrying the behavior around it, slowly manipulating events to take it back to its past host body, slowly puppeteering things behind the strings.
We are told sephiroth was the one in control, but sephiroth never truly existed - it was jenova reborn as sephiroth the entire time with sorta amnesia, and his exposure to the previous "body" triggered jenova to fully reawaken inside of him, which is why at times "sephiroth" the one you encounter doesn't recognize you. At that point the organism has "re-evolved" a human personality combined with its insytinct to suck a planet dry and move on. I sincerely believe this also happened with its previous host but when it was mummified its mind was completely destroyed, and it had to re-evolve back up from base instinct.
Thus the entire game, you are actually fighting jenova. Sephiroth is just a jenova host, who has applied a personality of what it was, or thought it was, as sephiroth on top of jenova - jenova is using it like a fake personality, but at the same time has learned about reality and about cloud. I think this parts complicated because i think the cellular structure of the previous "jenova" - the braindead one is overpowered by the new "sephiroth" jenova reincarnated. So ye sephiroth did overpower jenova- but theyre BOTH jenova - one is braindead cetra goddess jenova, the other is jenova reincarnated through sephiroth, who still has her oritginal personality, but another human element overlaid on top of its exiting personality - which is why the one true line from it "that's because, you're a puppet" is said by jenova itself, while its sock puppet personality of sephiroth is used to interact with the players because thats what they know her as. It knows the old "amnesiac" sephiroth called her mother and so it puppeteers this personality the whole time.
At no time does jenova ever stop pretending. It pretends to be sephiroth, who puppeteers its jenova parts from the braindead cetra goddess, who themselves morph into the form of sephiroth, but the entire time, the ENTIRE time, it is jenova pulling the strings. Sephiroth is a SHELL, an artificial construct that thought it was human until it remembered it was not, hence why seph's personality ENTIRELY changes after that and it abandons EVERYTHING it learned or knew - friends, emotions etc.