spoiler tag stuff like that, Lilo xD for the sake of those reading my profile
Well, the Memories of Kairi were what she got at first and what shaped her look, but then her own Memories no doubt kicked in, not to mention a lot more of Sora aside from just Kairi, so it adds up.
Which means that at the end, Xion being "based" on Sora's Memories of Kairi is all but redundant in light of how much else she has in her :\ and Riku and Namine being the ones to tell me Sora cares for her doesn't change much as my views about Namine's opinion in the matter are well known, and I can't blame Riku after 1) KH1 where Sora kept going "Kairi Kairi Kairi", 2) jealousy issues from before KH1, 3) spending what time he did with Namine.
One thing that does add up though is that Xion also had Sora's Memories of Castle Oblivion. So when Roxas fused eventually with Sora at the end, it would explain why Sora remembered Namine - through Roxas, Sora got those Memories back since Xion had them.