Okay so I can’t find the post but I remember long ago someone asking (if you’re still here bless your heart) about the scene in 358/2 where Xion dreams of the Riku and Zexion confrontation. Well I was going to ponder over it but I never really got a change to just sit down and do that. However I did get a chance to ask someone else their thoughts so I figured since it’s been a year I should just swallow my pride and post those.
There are two possible answers to this question
The Non-Fandom Answer: Nomura thought it would play into Xion’s conflict of ‘who am I’ (Not only do I have Sora’s memories; I have Riku’s as well? Who am I???). It also gives the player a sense of how Xion feels. When we first watch the scene we become confused and somewhat distraught because it seems to come out of no where. Similar, Xion is confused about who she is and distraught over what to do.
The Fandom Answer:
Only reason I can think of is that while on Destiny Island, she passed out and was picked up by Riku. Built as a puppet, her purpose was to sample memories of Sora’s/Roxas’s and make them her own strength and power. However, Sora is linked to a lot of people, so inturn Xion looks like Kairi + Sora. At the same time, she can look like Ven - yada yada yada.
Its fast, and not really explained much, but I honestly think she might have just sampled Riku while she was with him for that short period of time. If only due to the fact that she saw his memory of guilt from CO, one of the major instances that was causing him not to actually meet or run into Sora, until his darkness was fully under control. More or less a driving force, or one of the stronger memories that he went through in his determination that he needed to defeat his own darkness - Xion was somehow able to pick up on that, because she’s tied to Sora’s Heart as much as Riku is.
Or.. that’s about all i can figure