[voice]
Well, it's not sound because then it's basically impossible to always be aware if you and someone else come from the same world, as so many of those alternate dimensions could be similar enough that the differences wouldn't even matter to you or that person. You and this person could have all the same memories about each other, only in your world once the sky was cloudy on a day it should have been sunny in their world. It would mean you would never be able to do enough research and evaluating to find out.
Now this Roxas...are his memories of you just as you expect them to be, or are they also missing or altered? Because someone can perform the same act for different reasons.
[Heartless? He doesn't know what those are, but assumes they are probably a enemy in her world, since someone she knows had to fight them.]