The first time I learned of Xion (discounting the promotional trailer) was through a video of her (purposefully melodramatic, I concede, but it tugged my heartstrings nonetheless) death-scene on Youtube. Curiosity piqued, I decided to investigate her character more thoroughly.
And promptly discovered that she was so disliked that a (prominent) hate-club had been formed after her.
I found out the reasons soon enough. Xion unquestionably borderlines on - if not outright crosses over into - Sue-territory. 14th Organization member, wields a Keyblade, next-to-instant-BFF to Roxas and Axel, yada yada yada. Sounds like a typical fangirl self-insert, until you factor in her convoluted origins (which I'd say are cleverly devised, even if they're only there to justify her presence in the Org). Yet for this, her (already bland) character degenerates into mere plot device, which revolves around being the catalyst that drives Roxas' defection from the Org. As Xion-critics point out, the above purpose of hers is redundant (and perhaps even useless), since Roxas, continually deprived of information regarding his identity, would (probably have) left on his own accord, anyway. Her story is entirely self-contained, her design even co-ordinating with the convenient mind-wipe that explains her lack of mention in KH2.
Despite all this, I like Xion. While her personality may as well be a carbon copy of Naminé's (shy, reserved, meek), her humanity - like Roxas' - shines especially brightly in an environment governed by cold, unfeeling pragmatism. It is under her influence that Roxas and Axel become characters who are real and human and able to be sympathized with. Whatever others may say about her 'playing the martyr', I was struck by the sheer magnitude and nobility of her sacrifice. It holds no reward aside from saving Roxas (albeit temporarily, unbeknownst to her) and being a final act of defiance - she is doomed to oblivion for it. Not only will her precious, beloved friends fail to mourn her loss, it will be as though she had never existed to them. The courage and strength of heart required to overcome that is unthinkable.
I suppose some people would dismiss what I've found likable about Xion on the premise that she is a Sue. I would venture that they feel robbed that she, lousy Sue that she is, was the one who drove Roxas' (and Axel's) character development and had the plot centered around her (instead of Roxas or some other more deserving character). No doubt some hate her because she turns Roxas into a whiny brat who is constantly moping about her. In the latter case, I can only frown, as it seems they have missed the overarching principle of the matter!
What Days impressed upon me is the importance of emotions and the power of friendship. Roxas' character development exemplifies those. In no way did I find his outbursts (most frequently regarding Xion) whiny or childish; on the contrary, I thought them apt and powerful demonstrations of how someone would express concern over a dear friend. That Xion should be this friend is mere semantics. He would do no less for Axel, or anyone, for that matter, who had managed to earn themselves that special place in his (supposedly non-existent) heart. His interactions with Xion merely unearth the inherent kindness and loyalty within him, making him out to be the good person (and role model, too, I suppose, considering the main target audience) who he is.
That said, I don't begrudge anyone who hates Xion. The difference between my opinion and theirs is what makes her character palatable to me and not them. But I find it kinda pointless to expend so much negative energy over a reasonably well-designed character when not only are there far worse examples out there *coughcertainfemaleTwilightprotagonistcough*, but it is also much easier to content myself with enjoying the script (rather than agonizing over underwhelming character defects).