If we assume Ven's Keyblade somehow transformed into the KK, then it's Riku's.
Atta girl <3
Or she is Kairi related.
Alternatively, she's Aqua related but her age got messed with.
We still don't know about that.
All she'd have to do is live for about a year after BBS. Then Heartless and Nobodies would come to be, and she'd be free to turn into one herself.
yeah, i knew i didn't explain it as good as it went in my mind, oh well.
thanks for making it more clear!