From what I have seen, Xion is probably the most hated character in Kingdom Hearts. The main assumption is that she is a pointless character that does not amount to anything or have any overall purpose in the series. Majority people I have seen also do not like the retconned tie in for her either. I also feel that a reason she is disliked is because she was not shown in Kingdom Hearts II. With that being the case I feel she was already alienated as the newcomer of the group when days was released which only made it easier to dislike her more. However despite all of this, Xion is one of my favorite Kingdom Hearts characters. This is mainly based on my theory on the deeper meaning of what she really represents and how I relate to her. Throughout the Kingdom Hearts fanbase base the majority of the fandom has seemed to agree that Xion is useless and they do not like her for that since she has a pointless meaning throughout the series until the retconned. In my opinion, that’s the fucking point. Xion herself realizes that she is useless. Xion feels that she is useless, and Xion is made into a useless character throughout the years because you are supposed to feel the same way about her, the same way she feels about herself. This is similar to the theory of the overall meaning within the same game, 358/2 days for Roxas that the job and work of being an Organization XIII member and it being a chore and something he dislikes to make the player understand his struggle is exactly what I think could be the way Nomura wanted the player to feel- The same way he did with Xion. This makes Xion one of my favorite characters because I relate to her and how she feels about herself at times. She even felt that she was a mistake. I think Xion is pointless on purpose. As for the retcon, maybe Nomura decided to change this since nobody seemed to catch on or because she was hated on.