But from that to say he's a newborn with newborn emotions and reasoning would imply Xemnas should have the mindset of a ten years old, when he clearly picks up from where Terranort ended off.
Comparing Xemnas and Roxas is like comparing apples and oranges in this case since Xemnas did have memories from Terranort to base his personality and mindset off, not to mention that Xehanort himself may have had some remote-control as well.
Roxas had nothing
to base anything off from, neither Sora nor Ventus (the Sora memories Roxas gained came only later due to NaminÃ©'s messing around), that's why he diverges so much from his "original".
But he's not a newborn just because he became a Nobody-- he's a newborn in the sense that he has no memories of his own. Everyone else had memories from their previous lives, and that's part of why a lot of them thought Roxas's situation was odd.
Exactly, that's where the difference lies.
That doesn't make you like a newborn in terms of self-control and emotions. People experience memory loss without devolving that far and are still able to express themselves and reasoning.
Except that it isn't memory-loss
in Roxas' case but that he didn't have any memories to start with.
If you have no memories at all you do
start out largely similar to a newborn cause you don't have any experience of "expressing yourself" and "reasoning".
As for Xion, I don't hate her per se and I do find her character somewhat interesting, but it is undeniable that she was indeed quite the spotlight-stealing squad on her own and nearly everything in Days had either to revolve around her or connect to her somehow, delegating the other characters to mere statists in a game that was originally advertised as "Roxas' time in the Organisation" to which audiences reacting with expectations that it would be primarily about Roxas and the Organisation (as in the already known
members) is not really unfounded.
Roxas had practically zero agenda of his own because all active parts were "claimed" by Xion with all him doing throughout the game is whining left and right only for Xion here and Xion there, which some people can really only too easily compare with the dozens of self-insert fanfics written by "crazy" fangirls back in the early 2000s on FanFiction.net.
The development from his zombie-like subdued personality from the first part of Days to the more inquisitive, rather hardened personality we get to see glimpses on in KH 2 (I'm not talking about the brainwashed simulated personality given to him by NaminÃ© and DiZ) almost doesn't happen at all, he just suddenly falls into that personality shortly before the end.
Another beef I remember people who dislike Xion often bring up is that she was ever even foreshadowed or hinted at anywhere (the BBS trio did get teased beforehand verbally by Xigbar and due to secret endings) and even steals spotlight in scenes where there was no trace of a third person being involved, namely the Deep Dive scenes between Roxas and Riku, which Days somehow manages to again
make somehow about Xion rather than either Riku or Roxas.
A third issue I can recall is detractors stating that unlike i.e. NaminÃ©'s, Roxas' or TAV's tragic and sad story, Xion's was forced down the audiences throat every second scene and riddled so much with over-the-top drama that in the end it became annoying rather than touching.