Hello there! Sora player here! I just wanted to pop in and fill in some stuff! Roxas and Sora do know of her name, Roxas and Namine knowing more than Sora of course. Most recently Sora asked Namine more about Xion which was mostly that she was made from his memories and that she was Roxas' friend. He feels like he wants to protect her as much as he wants to protect Roxas. Sora is from Post-3D so he's seen her face from the last cutscenes of that game.

There is only one other cross-canon character that has "met" Xion as in she saw a glimpse of her when she traveled through Sora's heart during a player plot.

As for your question about apping Kairi, I think the general guideline is that you can't app a character that would or have interacted in the canon. So I'm not sure you could app both Xion and Kairi.

Hope to see you around!