In control: Jenova or Sephiroth

[Ⓐaron]

Sephiroth is indeed canonically in control but it is pretty suspicious that his goals come to align almost identically with Jenova's, which leads me to wonder whether that was the original intention or whether it's a retcon. I like the "symbiosis" idea.

You can't edit posts until you make 10 of them, btw.

 

[Starling]

As has been said, Sephiroth being in control is canon. He willingly and knowingly embraced his role as Jenova's son, hence why he decided to do what it tried to do. He actually didn't decide on the god part of his plan until he learned a bunch of stuff in the lifestream.

In Nibelheim, he spent about a week looking into stuff, presumably without eating or sleeping, before he snapped. At that point, he thought he was a Cetra and blamed humans for their extinction, deciding they all needed to die for their treachery. Once he finds out Jenova isn't a Cetra, he decides to destroy the planet and everything on it, since that's basically what Jenova wanted to do and he presumably blames them for what happened to her.