All as One

Okay, so... Chapter 4 of Season 5 has basically all but said Rian is Rain's son, so I am gonna do this. Rian is bonded with the Ultima of his world, which was born after the Espers were killed by the King of Chaos, their powers merging and forming into Ultima, taking on a form similar to that of Ultima Risen from XVI. His Semi-Primed form is similar to Rain's, though he gains four of the Six wings from Ultima Risen. His fully Primed form is basically just Ultima Prime/Risen, through and through. Rian bonded with Ultima years after the Fina and Rain of his world had died, taking him along with in order to execute their plan.