There are some fair criticisms on Xion's character and how she was planned after KHII had already came out. Some I even agree with. My thoughts on Xion? Xion is a weird freak of a character...and I love it. She was designed to be Roxas, looks like a goth Kairi, and has Sora's memories poured into her. I don't know why I love the fact that she's so needlessly convoluted, but it never bothered me in a series where we have a organization made up of 13 dead bodies. Seriously the puppet fits right in.
I've always been aware of the hate this character got, but she never bothered me. On the contrary, it was her inclusion that made me want to play Days to see what she was, which lead me to loving Roxas. Roxas went from being one of my least favorite characters in the franchise to becoming my number 1 favorite, just because I was curious on what a Xion was.
I also adore her friendship with Roxas and later Axel, and how it didn't feel forced on me. They also showed me the friendship rather then just telling me she was friends with them. I appreciate that. My favorite Xion moment's don't even include her death. I love the missions she went on with Roxas. It's one of the few times I loved a Agrabah segment in Kingdom Hearts. I loved watching Roxas and Xion meet Genie, dodge all his questions and turn them back onto him, and once they get their answers they leave him in the dust without so much as a "goodbye" or "thank you"
I like how the friendship with Roxas evolved. Like when Xion can't use a Keyblade and Roxas says he'll cover for her, she doesn't outright tell Roxas not to trouble himself for her, she jumps at the chance at hiding behind him. It comes off selfish but then I remember she doesn't have a heart so she can't tell the difference anyway and it makes me laugh. Compare that to the end of the game where she stops caring about herself and only cares about Roxas and Axel and it's a big change in character. Because they do care for one another, and it shows when they go on missions together. I love cheesy friendships in KH. That's my main reason I love this series. Everyone should become a friend!
It also amuses me that Xemnas' big plans for a Sora replica all fail because Roxas extended a hand of friendship to his murder puppet. Another thing Xion has terrible AI. I always imagine when you first have her as a partner, she's secretly trying to get you killed so she can absorb you faster, as that was what she was suppose to do with Roxas. But then she became a person and it ruined that plan, and she attacks a little more...a little lol.
I probably enjoyed Xion far more then I should have. But I can't imagine playing Days without the little puppet in it. One complaint about Xion is she doesn't add anything to the overall plot of KH. That complaint is pretty correct, but for me it's another positive. I like that her story is self contained. It doesn't really bleed into the overall plot like "Oh, this is why Sora can wield a Keyblade!" "Oh, this is why Kairi is magical!" "Oh, this is why Xehanort can do this!" "Oh, this is why this character looks like that character, instead of this character!" 358/2 Days feels optional and I always respected that about Xion.
So in short Xion is a weird freaky character and I love weird freaky characters. Her ability to change into other characters also makes me want to see everyone in the franchise to meet her just to see what they see her as. Did Xion need to exist? Nope. Would Roxas's story have been fine without her? Probably. Am I glad they made her? Yes. Xion takes the lore of Kingdom Hearts, puts it under her coat and lets out a big smelly dutch oven fart on it XD