well i think that Xion was made from Sora's Memories that got leaked when Sora turned himself to a heartless, some memories went to Roxas just as mere images not as memories because Sora gained a full self again very quickly,, same for Xion (image memories),, but remember how Namine could manipulate ALL the memories of Sora and all those who are connected to him, and Riku is one of the persons who is connected to Sora more than anyone (regarless Kairi xD),, maybe thats why Xion saw THAT scene of Zexion and Riku,, because she got some memories of Riku too in herself and she reflected it from Riku because he is REALLY that connected to Sora (same way Roxas could talk to Kairi in KHII,, 'cause she is REEALLY REALLY connected to sora :3 :B hahaha well its just my opinion =P
well umm I really really think that (actually I think of it as Xion Talks to Riku telling him to stop Roxas during the fight) that MAYBE the one who threw Oblivion at Riku was actually Xion, to give him like an advice he must stop Roxas at any cost knowing he'll just get destroyed going on his own...That's why maybe when he took oblivion he suddenly had some random memories of her dafing away... And since Xion IS Sora AND Roxas maybe she found the way to do that (:
well I think of that while seeing for the 4th time the fight 8-) haha ^^U
