Mythra because I felt that she was the OG. Obviously context changes when you beat the game.

But she's like 12. o_O

Zeke

She isn't. Much much older.

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.

Sauce? I don't recall them specifying her age. Definitely looks like a minor though.

Well when Pyra is technically like 500 years old looks might not be the best measure for age in this game...

For how long they've existed for? Sure. For how mature and developed their body is? Naw.

Mythra is blond, so she's already disqualified as a waifu.

About the same age if not older than Rex. So 18. Or just 500 years.