Tidus' new girlfriend is called Jennah Ovarium. Mid-game, it is revealed that she was once a summoner who tried to defeat Sin, and is the one controlling the new Sin. She had a twin brother who was her guardian, who gave up his life to become the Final Summoning. Before she could fight Sin, Yuna and co defeated Sin. Also it is revealed Jecht was their father. She broke up Yuna and Tidus in a convoluted revenge scheme. When she learns Tidus is her half-brother, she goes mad.
For the final battle, she merges with Sin. In the ending, broken and battered, she latches herself unto Shinra's space shuttle and takes it over, making it crash into an unknown planet. Calling herself Jenova, she ingratiates herself with the natives called Cetra, destroying them from within. Shinra's descendants use the wreckage of the shuttle and the technology contained within to create proto-Midgar. A couple hundred years later, a young mercenary called Cloud Strife stows aboard a train headed towards Reactor 7. Game ends. FF7 remake is announced at the next E3.
Am I close?