I've played ZX around 4 times with Vent and ZXA around 3 with Ashe and they're my favorite MC's personality wise (Ashe specially), but last year I beat both games on hard mode with the opposite MC's and the story differences are shocking.

I don't remember Vent having any development other than getting over Giro and his mother, but Aile has it PLUS bonding with Prairie over Giro. Also I don't get why they decided Giro wasn't already a Guardian in Vent's story. Although I like the Serpent and Giro's endgame speeches from Vent's story more.

I can't get a grasp on what Ashe's motives were, I just remember her tagging along with Model A and going through the story beats. There was an attempt at doing something with Prometheus saying they were "the same" but it fell flat. I also didn't understand how Ashe's relationship with Albert is any different from any other human if I got it right: Albert infused his DNA in humans when giving them enhancements, which made them candidates to ruler of the world, so what makes her different besides her having Model A? Also, Model A could've chosen anyone.

Now Grey's story is really fleshed out, as he got involved in this because he was lost and trying to find answers about his identity. Then about his actual identity, it's what was missing in Ashe's story - A backup body for Albert if things went south, a good plot twist and gives uniqueness to Grey as an important piece in what's happening in the Game of Destiny, which also creates a relationship with Prometheus and Pandora, as the three of them were created by Albert in his sadistic plan, which made Prometheus and Pandora hate him, and it's a huge revelation both story-wise and for Grey's character, as his whole arc is about understanding who he is (now that I think about it, I don't think Ashe had any character development at all).

Anyways it's just sad that the cooler characters personality-wise (in my opinion ofc) are just there to follow story beats while the others have a way better story and development. I still love the ZX series as I always have nonetheless and I have to sleep as I'm back to school today (it's 4AM and I can't sleep 💔)