Despite everything that the OP says, while true, I think Rex would still choose Pyra, he's just crazy for her, even if she's still Mythra and whatever. To begin with, she's the reason Rex started the adventure, he's also grateful that she resurrected him, and he more or less developed a strong bond with her, although still juvenile. You know, when they're in the campfire at the end of Chapter 2, when they play with each other in Garfront's camp in C3 and after Mythra awakes next to Rex. Mythra is also mostly relegated to silly situations when it comes to showing her person to Rex, the aforementioned bed joke, the one-eyed moster thing, the "sleeping on your lap" scene. It's even more juvenile than Rex's relationship with Pyra, so I don't doubt he would've chosen Pyra as Pneuma's other name.
Despite that, I chose Mythra, though Pythra/Mothra would've been cool, and you know that Nia is the only correct answer.
My bet with The_Liquid_Laser: I think the Switch won't surpass the PS2 as the best selling system of all time. If it does, I'll play a game of a list that The_Liquid_Laser will provide, I will have to play it for 50 hours or complete it, whatever comes first.
