[He pauses as he considers what to say. This is hard.]

We didn't know about it at the time, but a while ago, they forced Namine to change Sora's memories. She has that power. Then when he went to sleep so she could change them back, some of them got scattered and found their way to Xion. Other ones came through me, to her, because I'm his Nobody.

I don't know how it works, but that's what they did. They wanted her to absorb Sora's memories and then absorb me because it would give them control over his power. Xion figured it out first and refused to let that happen.