KHIII's biggest slap in the face to me was doing Kairi and Naminé dirty. They were both used for trophies despite that it didn't need to be the case. Out of Naminé's connections and bonds, like Roxas and Kairi, Nomura used Riku Replica? Because he has such affection for this character now? So screw all with Naminé right. It's
Riku Replica that he's going to focus on getting closure. And he utilized Naminé as a tool for it (the girl didn't even get a say in all of this) when she...when did she care about him again? My, I don't believe there's any build up to this, oh gee.
Larxene, Kairi, and Naminé. All of them got relegated to be male fodder devices. Because what
else can motivate women, right? Nomura sure as hell doesn't know! Even Strelitzia. She's
worse than Kairi if that's even possible, such a non-character. And with a Keyblade. She existed to humanize Marluxia and give him onii-chan angst.
I’m not gonna argue the “useless” part but I never understood the “story could’ve happened without her” argument. Like it happened because they wanted to create and use a character as a catalyst.
Not only that, that argument opens a whole can of worms in arguing everyone’s importance to the tiniest details.
Xion is a character that's smacked dab in a "mid-quel" of sorts. 358/2 Days is a self contained story, and Xion's own arc is basically KHII Roxas 2.0 [Expanded Edition]. You even have Riku as the supportive "Naminé" role. The concepts and theme shas been covered by
both her friend and Riku's replica.
Furthermore,
with or without Xion, KHII would have happened the same way concerning Roxas. Because it
did happen before her conception/before she existed. He left the Organization, had an out with Axel, got captured, and is still the piece missing to wake up Sora. "Days" could've been anything.
Aside from cameos after, Xion does not affect anything else either. She's simply tied to the relationship with Axel and Roxas.
Xion isn't like other characters in which the series would have to do an overhaul or change certain aspects if they were taken out because they have had a hand in events
beyond their introductory game. This includes Kairi (yes, yeees), Riku, Naminé, Roxas, Aqua, Axel/Lea, and Terra. Ventus can be debated, but I don't care enough about him to put effort in his defense. Someone else can though.