While Sephiroth's persona is the dominate one at the forefront, Jenova's instinct and will lives on through him, driving him to destroy/eat/consume the planet. You kill him, er "them" at the end of the game but their essence is still floating in the lifestream. Hence why in advent children, when that kid takes those Jenova cells into his body, Sephiroth comes out.

There is a lot of discussion about Jenova not being sentient and if it's her or Sephiroth in control. It's true she never really talked, but there is no doubt there is some form of intelligence there. She manipulated and twisted Sephiroth's mind to the point where he wanted the same thing she did, to consume the world and ride it like a vessel through the stars consuming other planets. Even before she attached herself to him and he seemingly took control. Even if she is just a parasite, she successfully inflicted her will upon a host, even if the host still believes what it is doing is by it's own free will. Sephiroth was a calm and tranquil scholar before her influence.

Gotta take issue with these. The Ultimania guide makes it clear what's up with Jenova's situation. Jenova is not intelligent, at all. Jenova is a virus, it has two impulses (1. infect more life forms to spread Jenova 2. reunite). Sephiroth's great will allows him to override those impulses and make Jenova's cells do his bidding.

Jenova is an epidemic, not an individual. Sephiroth never 'intended to ride to other stars and planets', not initially. What Sephiroth wanted to do was grievously maim the planet, then dive into the resulting wound as the entire Lifestream gathers to heal it. Consume the Lifestream, become a god, and remake the world according to his desires.

The "turn the planet into a vessel" plan is just after Cloud cut him up and he went insane. Or it's Kadaj's delusional fever dream after he gets high on Jenova.