She's literally the 'new' best friend of two already established characters and is a main character now, is ""tragic"", and has a "dramatic" and "sad" story.
Case in point, this description can be assigned to practically any character in the series after the first game.
She even has a Keyblade, which is the weapon only limited people can use.
Sounds pretty self-inserted to me.
Again, case in point: what about Namine and Chain of Memories? New best friend of Sora and Riku, dramatic and sad story, ends in pretty tragic fashion.
Do you even know what a Mary-Sue is?
An unhelpful catch-all descriptor for a writing trope which, with enough selective reading of the targeted material, can be applied to practically any sequel-specific character in the history of literature.
Given that, I'm using the perspectives provided in this thread to ground the discussion, because otherwise it's just too nebulous.
A female fanfiction character who is so perfect as to be annoying. The male equivlalent is the Marty-Stu. Often abbreviated to "Sue". A Mary Sue character is usually written by a beginning author. Often, the Mary Sue is a self-insert with a few "improvements" (ex. better body, more popular, etc). The Mary Sue character is almost always beautiful, smart, etc... The Mary Sue is overpowered. In short, she is the "perfect" girl. The Mary Sue usually falls in love with the author's favorite character(s) and winds up upstaging all of the other characters in the book/series/universe. There are several main types of Mary Sue
So focusing on the main definition, this doesn't jive with Xion's character at all. Xion isn't intended to be "perfect" or even necessarily "good": she spends the majority of the game confused and prone to bitterly lashing out at the people around her, essentially attempting (wrongly) to work out her troubles as a one-woman island who actively isolates from the other characters, investing only in Riku because she sees in him a reflection of her own journey (Xion's thematic power as a mirror character is paramount) and a kindred spirit. The questions of Xion's appearance is overtly deconstructed, given that her nature is to reciprocate the vision of others so her "beauty" can only be conferred upon her: even her appearance doesn't belong to her. Overpowered? She only has access to that super rare "Keyblade" (which has been or will be wielded by every single KH original main good guy and several Disney tie-ins by the time KH3 rolls around) through her connection to other, more powerful characters: she's consistently shown getting her ass whooped even by characters who don't have a Keyblade. Upstaging? This is superficially true, but a closer reading indicates that the whole point of Xion's character is to reframe and deepen our understanding of existing character dynamics and the natural logic of the KH universe, to put the greater story that is occurring around her into perspective.
Victim!Sues: The Victim!Sue is your whiny, wimpy, pathetic female character who can't seem to do much of anything except cry and get herself into trouble that the romantic interest of the fic has to rescue her from.
The bold part is basically a description of Xion.
2: Can't seem to do much of anything except cry
And this is why I say the descriptor is nebulous and unhelpful. You can't have it both ways, but as it happens, this completely contradictory version of the trope you just listed is no more relevant here. First of all, it lacks context: let's go back to Namine. She starts CoM having been kidnapped, spends the entirety of the game literally taking on the role of romantic interest of the main character, arguably accomplishes nothing of her own volition, bemoans her circumstances and uses them to justify her melancholic so-sad attempt to entrap SDG and then whines to them about how even that
wasn't the right choice, and then basically sits around and begs everyone to forgive her for the rest of her character arc.
Obviously that is a totally unbalanced, surface-level reading of her character which doesn't take any of the particulars of her circumstances and the real strengths of her character into account. But that's how people read Xion. Doesn't do much of anything? Xion abandons the Organization light years before Roxas ever got enough of a clue or the gumption to do so, then runs away again
when they try to bring her back. She spends the game actively investigating her own origins and trying to navigate a world that is unfamiliar and essentially hostile to her, advocating as best she can for her own right to exist while being forced to reckon with the undeniable truth that her presence runs contrary to the greater good. After a game's worth of contemplation, she then uses this knowledge to summon the courage to make the necessary sacrifice in order to secure a safer future for the people she cares about. A victim?
: Did I do this to you?
: No. It was my choice to go away now.
The whole of Xion's story is her journey to reclaim her agency and identity, working against the tides of a world that won't allow her to have a happy ending where she can be the person she wants to be with the people she wants to be with and making tough choices about what that ultimately means for her and for them. But the great thing about Days
(like CoM before it) is that it's chock full of those imperfect decisions and genuine character flaws which prevent the story from reaching a state of straightforward conclusion: Xion sets the precedent for what Roxas needs to do, leading by example, but because Roxas is his own character who has had his own experiences over the course of the game, it drives him in a different and unintended direction, furthering the thematic wedge between them as characters: Xion sees the need to act and does so-- and therefore works in tandem with nature and a universal understanding of destiny/fate without submitting to it (it's important to recognize that her sacrifice contains many, many echoes of Sora's in KH1); Roxas sees the same situation and digs his heels in, conversely decrying the inevitable endgame yet ultimately still finding himself subject
to it (literally flat on his back in front of Riku).
: You and I may both be exceptional Roxas, but I don't think we're quite the same. (Day 151)
This doesn't even touch on the constant use of "time" as a refrain (the visual symbolism of the ice cream as a marker), Xemnas's own repeated failures of imagination and how that contextualizes him as a character ~and the best KH villain~ (indeed, one who knows nothing can understanding nothing), or the excellent work done to bridge Axel's extremes in personality which puts that old KH question of what a real friendship is to the ultimate test. Xion made all of that possible as a matter of presentation, and while it's obviously true the writing team could have gone about things any old way they wanted-- that's the nature of storytelling. Everything could always be different.